Polarizing plate having a stretched film on a side thereof and liquid crystal display employing the same

ABSTRACT

A polarizing plate used in a vertical alignment mode liquid crystal display comprising a liquid crystal cell is disclosed, wherein the polarizing plate comprises a polarizing film and a mixed fatty acid cellulose ester film with optical biaxiality provided between the polarizing film and the liquid crystal cell, and wherein the mixed fatty acid cellulose ester film has a retardation in plane R 0  of from 31 to 120 nm, and a retardation in the thickness direction Rt of from 60 to 300 nm, in which R 0  is represented by the following formula (1) and Rt is represented by the following formula (2):
 
 R 0=( nx−ny ) ×d   formula (1)
 
 Rt ={( nx+ny )/2− nz}×d   formula (2)

FIELD OF THE INVENTION

The present invention relates to a polarizing plate employing as aprotective film a film used for optical purposes having a phasedifference function and a liquid crystal display employing thepolarizing plate.

BACKGROUND OF THE INVENTION

A liquid crystal display operates at low voltage as well as low powerconsumption, and further, can be directly connected to an IC circuit.Specifically, it is possible to decrease its thickness. As a result, itis widely employed in word processors, personal computers, and the like,as the display. The basic structure of said liquid crystal display issuch that polarizing plates are provided on the both sides of the liquidcrystal cell.

A liquid crystal display has characteristics in that it operates at lowvoltage as well as at low power consumption, and decreases itsthickness, which other displays do not have. Problem of the liquidcrystal display is one in which a viewing angle is small. Solution ofthe problem has been eagerly required, and technique to solve thisproblem has been developed. As the technique, there has been proposed anoptical compensation film comprising an optically anisotropic layercontaining a discotic liquid crystal compound obliquely oriented, whichis provided on a polarizing plate of a TN (TN-TFT) mode liquid crystaldisplay. However, further wide viewing angle in, for example, has beenincreasingly demanded year by year.

As one of the means solving the above problems, there has been proposeda liquid crystal of type different from a TN or STN type liquid crystal.A vertical alignment (VA) mode liquid crystal, for example, a negativeliquid crystal having a negative dielectric anisotropy, has beendeveloped, in which when no voltage is applied, liquid crystal moleculesare oriented vertical to the orientation plate, while when voltage isapplied, they are oriented parallel to the orientation plate. In the TNor STN type liquid crystal, when no voltage is applied, liquid crystalmolecules are oriented parallel to the orientation plate, while whenvoltage is applied, they are oriented vertical to the orientation plate.A VA mode liquid crystal display comprises a liquid crystal cell with avertical alignment mode in which when no voltage is applied, liquidcrystal molecules are oriented vertical to the orientation plate, whilewhen voltage is applied, they are oriented parallel to the orientationplate.

This VA mode liquid crystal display comprises a liquid crystal cell witha vertical alignment mode in which when no voltage is applied, liquidcrystal molecules are oriented vertical to the orientation plate, whilewhen voltage is applied, they are oriented parallel to the orientationplate. As a result, in such a liquid crystal display, black is displayedas genuine black, and contrast increases. Particularly in a VA modeliquid crystal display multi-divided, it has been possible to obtain aviewing angle of 160° at the upper, lower, left and right portions ofthe display. However, as the size of a liquid crystal display increases,further increase of the viewing angle, particularly the viewing angle inthe oblique direction of 45° (in the direction of 45°, 135°, 225°, 315°)has been increasingly demanded.

In order to increase the viewing angle of the VA mode liquid crystaldisplay, the present inventors has made an extensive study on apolarizing plate protective film. During the course of theinvestigation, and as a result, they have found that when a film isemployed in which an retardation value (Rt value) in the thicknessdirection, showing anisotropy in the in-plane direction as well as inthe thickness direction, is adjusted to a positive value, that is, aso-called C-plate in which anisotropy in plane is adjusted to a minimum,and a retardation in the thickness direction to a specific range, isused, the viewing angle of the VA mode liquid crystal display furtherincreases (Japanese Patent O.P.I. Publication No. 2001-188128). However,this method could not prevent the thickness from increasing, although itis smaller in the thickness as compared with those in which apredetermined Rt is obtained by laminating plural conventionalpolarizing plate protective films.

It is known that insertion of a phase difference plate with opticalbiaxiality of a resin such as polycarbonate between a polarizing plateand a liquid crystal cell increases the viewing angle described above,the optical biaxiality being provided by biaxially stretching the phasedifference plate, and for example, “VAC film” produced by SumitomoKagaku Co., Ltd. is available on the market. However, a phase differenceplate of a resin such as polycarbonate has problems in uniformity ofphase difference or in transmittance, and is required to be laminatedonto a polarizing plate every piece, which results in problems inproductivity or cost of manufacture.

An optical film, particularly a polarizing plate protective film isrequired to have resistance to heat or humidity, which prevents itsshrinkage or deterioration due to moisture absorption of a polarizer athigh temperature and high humidity, and separation the polarizing platefrom a glass plate of the liquid crystal cell due to deterioration ofthe adhesive used. Further, the optical film is required to have hightransparency, high strength, or ease of handling.

Further, when cellulose ester film is produced by dissolving celluloseester and casting the resulting solution, cellulose ester in which theesterification has been insufficient tends to remain in the film asinsoluble foreign materials. When such a film is incorporated to aliquid crystal display element, the insoluble foreign materials causesdefects that disturb the polarizing state to emit abnormal light due toits difference in the refractive index from cellulose ester film,so-called defects due to luminescent foreign materials. These defectshave been less problematic in a TN mode (TN-TFT) liquid crystal display,which is an optical rotatory mode, but it has been found that they areproblems to be solved in a VA mode liquid crystal display, which is abirefringence mode.

Japanese Patent O.P.I. Publication No. 9-90101 proposes that casting canbe carried out employing a solvent other than chlorine-containinghydrocarbons as the solvent, where incorporating a specific amount of anacetyl group or a propionyl group to cellulose to obtain a celluloseester, selection range of the solvent of the cellulose ester increases,and also proposes that fatty acid cellulose esters having a lowretardation value in both in-plane and thickness directions are used forthe purpose such that high contrast of a TFT mode or FSTN mode liquidcrystal display realizing high contrast is not jeopardized.

However, when used in the VA mode liquid crystal display, any proposalsdescribed above are not satisfactory in view of resistance to heat andhumidity, the number of foreign materials occurring or increase inviewing angle. Further improvement is required.

SUMMARY OF THE INVENTION

The present invention has been made in view of the above. Accordingly,one aspect of the present invention is to provide a polarizing platehaving high resistance to heat or humidity, high strength, and minimizedforeign materials, and a liquid crystal display employing the polarizingplate. Another aspect of the present invention is to provide apolarizing plate giving wide viewing angle and a liquid crystal displayemploying the polarizing plate.

BRIEF EXPLANATION OF THE DRAWINGS

FIG. 1 is an illustration showing the constitution of the liquid crystaldisplay of the invention.

FIG. 2 is an illustration showing the cellulose ester film in theinvention manufactured according to a cast film manufacture process.

FIG. 3 is an illustration showing the constitution of the polarizingplate of the invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention can be attained by the following constitutions:

1. A polarizing plate used in a vertical alignment mode liquid crystaldisplay comprising a liquid crystal cell, the polarizing platecomprising a polarizing film and a mixed fatty acid cellulose ester filmwith optical biaxiality provided between the polarizing film and theliquid crystal cell, wherein the mixed fatty acid cellulose ester filmhas a retardation in plane R0 of from 31 to 120 nm, and a retardation inthe thickness direction Rt of from 60 to 300 nm, in which R0 isrepresented by the following formula (1) and Rt is represented by thefollowing formula (2):R0=(nx−ny)×d  formula (1)Rt={(nx+ny)/2−nz}×d  formula (2)wherein nx is refractive index in plane of the film in a first directiongiving a maximum refractive index, ny is refractive index in plane ofthe film in a second direction normal to the first direction, nz isrefractive index in the thickness direction of the film, and d isthickness (nm) of the film.

2. The polarizing plate of item 1 above, wherein the mixed fatty acidcellulose ester film has a retardation in plane R0 of from 31 to 60 nm,and a retardation in the thickness direction Rt of from 90 to 200 nm.

3. The polarizing plate of item 2 above, wherein the mixed fatty acidcellulose ester film has a retardation in plane R0 of from 31 to 50 nm,and a retardation in the thickness direction Rt of from 110 to 150 nm.

4. The polarizing plate of item 1 above, wherein the mixed fatty acidcellulose of the mixed fatty acid cellulose ester film has an acetylgroup and a propionyl group in the ester bond.

5. The polarizing plate of item 1 above, wherein the mixed fatty acidcellulose ester of the mixed fatty acid cellulose ester film has anacetyl group and a butyryl group in the ester bond.

6. The polarizing plate of item 1 above, wherein the mixed fatty acidcellulose ester film has a thickness of from 30 to 110 μm.

7. The polarizing plate of item 1 above, wherein the mixed fatty acidcellulose ester of the mixed fatty acid cellulose ester film has a totalacyl substitution degree of from 1.2 to 2.8.

8. The polarizing plate of item 4 above, wherein the mixed fatty acidcellulose ester has a total acyl substitution degree of from 1.5 to 2.3,and a propionyl substitution degree of from 0.6 to 1.2.

9. The polarizing plate of item 7 above, wherein the mixed fatty acidcellulose ester has an acetyl substitution degree of less than 2.0.

10. The polarizing plate of item 8 above, wherein the mixed fatty acidcellulose ester has an acetyl substitution degree of less than 2.0.

11. The polarizing plate of item 1 above, wherein cellulose triacetatefilm is provided on the surface of the polarizing film opposite themixed fatty acid cellulose ester film.

12. The polarizing plate of item 1 above, wherein the angle formedbetween the orientation direction of a dichroic substance in thepolarizing film and the direction giving a maximum refractive index inplane of the mixed fatty acid cellulose ester film is in the range offrom 80° to 100°.

13. The polarizing plate of item 1 above, wherein the angle formedbetween the orientation direction of a dichroic substance in thepolarizing film and the direction giving a maximum refractive index inplane of the mixed fatty acid cellulose ester film is in the range offrom 0 to 10°.

14. The polarizing plate of item 1 above, wherein the mixed fatty acidcellulose ester film is provided on a first surface of the polarizingfilm, and a cellulose ester film is provided on a second surface of thepolarizing film opposite the mixed fatty acid cellulose ester film, thestretching direction of the mixed fatty acid cellulose ester film beingin accordance with that of the cellulose ester film provided on thesecond surface of the polarizing film.

15. The polarizing plate of item 14 above, wherein the cellulose esterfilm provided on the second surface of the polarizing film has beenstretched in the transverse direction during or after manufacture.

16. The polarizing plate of item 15 above, wherein the cellulose esterfilm provided on the second surface of the polarizing film has beenstretched at a stretching magnification of from 1.01 to 1.2.

17. The polarizing plate of item 16 above, wherein the cellulose esterfilm provided on the second surface of the polarizing film has beenstretched at a stretching magnification of from 1.05 to 1.15.

18. The polarizing plate of item 14 above, wherein the cellulose esterfilm on the other side of the polarizing film is a triacetyl celluloseester film.

19. The polarizing plate of item 1 above, wherein when the mixed fattyacid cellulose ester film is arranged in a crossed Nicol state, andluminescent foreign materials are observed, the number of luminescentforeign materials with a size of from 5 to 50 μm is not more than 150per 250 mm² of the film, and the number of luminescent foreign materialswith a size exceeding 50 μm is zero per 250 mm² of the film.

20. The polarizing plate of item 1 above, wherein the mixed fatty acidcellulose ester film is manufactured by a process comprising the stepsof giving optical biaxiality to a long length mixed fatty acid celluloseester film, and winding the resulting film around a spool to obtain along length roll film.

21. The polarizing plate of item 1 above, wherein the polarizing plateis manufactured by a process comprising the steps of giving opticalbiaxiality to a long length mixed fatty acid cellulose ester film,treating the film to give a maximum refractive index in the transversedirection of the film, winding the resulting film around a spool toobtain a roll film, providing a polarizing film containing a dichroicsubstance, and laminating the polarizing film onto the roll film.

22. The polarizing plate of item 21 above, wherein the opticalbiaxiality giving step is a step in which the long length mixed fattyacid cellulose ester film is stretched in the transverse direction ofthe film when it has a residual solvent content of not less than 10% byweight during manufacture.

23. The polarizing plate of item 1 above, wherein the mixed fatty acidcellulose ester film contains 1 to 30 parts by weight of at least oneplasticizer selected from the group consisting of a phosphate compound,a fatty acid ester compound, a citrate compound, and a phthalatecompound based on 100 parts by weight of the cellulose ester.

24. The polarizing plate of item 1 above, wherein the mixed fatty acidcellulose ester film contains 0.005 to 0.3 parts by weight of particleshaving an average particle size of not more than 0.1 μm, based on 100parts by weight of the cellulose ester.

25. The polarizing plate of item 1 above, wherein the mixed fatty acidcellulose ester film contains 0.8 to 3.0 parts by weight of a UVabsorber based on 100 parts by weight of the cellulose ester.

26. A liquid crystal display comprising a polarizing film A, apolarizing film B, and a vertical alignment mode liquid crystal cellprovided between the polarizing films A and B, one or more celluloseester films A being provided between the polarizing film A and theliquid crystal cell, and one or more cellulose ester films B between thepolarizing film B and the liquid crystal cell, wherein 31 nm≦Σ R0≦120nm, and 60 nm≦Σ Rt≦300 nm, in which Σ R0=Σ R0(A)+Σ R0(B), and Σ Rt=ΣRt(A)+Σ Rt(B), Σ R0(A) being the sum of retardation in plane of each ofthe cellulose ester films A, Σ R0(B) the sum of retardation in plane ofeach of the cellulose ester films B, Σ Rt(A) the sum of retardation inthe thickness direction of each of the cellulose ester films A, and ΣRt(B) the sum of retardation in the thickness direction of each of thecellulose ester films B.

27. The liquid crystal display of item 26 above, wherein at least one ofthe cellulose ester films A and B is a mixed fatty acid cellulose esterfilm having a retardation in plane R0 of from 31 to 120 nm, and aretardation in the thickness direction Rt of from 60 to 300 nm, in whichR0 is represented by the following formula (1) and Rt is represented bythe following formula (2):R0=(nx−ny)×d  formula (1)Rt={(nx+ny)/2−nz}×d  formula (2)wherein nx is refractive index in plane of the film in a first directiongiving a maximum refractive index, ny is refractive index in plane ofthe film in a second direction normal to the first direction, nz isrefractive index in the thickness direction of the film, and d isthickness (nm) of the film.

28. The liquid crystal display of item 27 above, wherein the mixed fattyacid cellulose ester has an acetyl group and a propionyl group or abutyryl group in the ester bond in the molecule.

29. The liquid crystal display of item 27 above, wherein the mixed fattyacid cellulose ester film has a total acyl substitution degree of from1.2 to 2.8.

30. The liquid crystal display of item 27 above, wherein the mixed fattyacid cellulose ester film has a total acyl substitution degree of from1.5 to 2.3, and a propionyl substitution degree of from 0.6 to 1.2.

31. The liquid crystal display of item 27 above, wherein the angleformed between the orientation direction of a dichroic substance in thepolarizing film and the direction giving a maximum refractive index inplane of the mixed fatty acid cellulose ester film is in the range offrom 80° to 100°.

32. The liquid crystal display of item 27 above, wherein the angleformed between the orientation direction of a dichroic substance in thepolarizing film and the direction giving a maximum refractive index inplane of the mixed fatty acid cellulose ester film is in the range offrom 0 to 10°.

33. The liquid crystal display of item 27 above, wherein the mixed fattyacid cellulose ester film is provided on a first surface of thepolarizing film, and a cellulose ester film is provided on a secondsurface of the polarizing film opposite the mixed fatty acid celluloseester film, the stretching direction of the mixed fatty acid celluloseester film being in accordance with that of the cellulose ester filmprovided on the second surface of the polarizing film.

34. The liquid crystal display of item 27 above, wherein the celluloseester film provided on the second surface of the polarizing film hasbeen stretched in the transverse direction during or after manufacture.

35. The liquid crystal display of item 27 above, wherein when the mixedfatty acid cellulose ester film is arranged in a crossed Nicol state,and luminescent foreign materials are observed, the number ofluminescent foreign materials with a size of from 5 to 50 μm is not morethan 150 per 250 mm² of the film, and the number of luminescent foreignmaterials with a size exceeding 50 μm is zero per 250 mm² of the film.

36. The liquid crystal display of item 27 above, wherein the mixed fattyacid cellulose ester film contains 0.005 to 0.3 parts by weight ofparticles having an average particle size of not more than 0.1 μm, basedon 100 parts by weight of the cellulose ester.

37. The liquid crystal display of item 26 above, wherein 60 nm≦Σ R0≦120nm, and 100 nm≦Σ Rt<300 nm.

38. A vertical alignment mode liquid crystal display comprising apolarizing plate and a liquid crystal cell, the polarizing platecomprising a polarizing film and a mixed fatty acid cellulose ester filmwith optical biaxiality provided between the polarizing film and theliquid crystal cell, wherein the mixed fatty acid cellulose ester filmhas a retardation in plane R0 of from 31 to 120 nm, and a retardation inthe thickness direction Rt of from 60 to 300 nm, in which R0 isrepresented by the following formula (1) and Rt is represented by thefollowing formula (2):R0=(nx−ny)×d  formula (1)Rt={(nx+ny)/2−nz}×d  formula (2)wherein nx is refractive index in plane of the film in a first directiongiving a maximum refractive index, ny is refractive index in plane ofthe film in a second direction normal to the first direction, nz isrefractive index in the thickness direction of the film, and d isthickness (nm) of the film.

101. A polarizing plate used in a vertical alignment mode liquid crystaldisplay comprising a liquid crystal cell, the polarizing platecomprising a polarizing film and a mixed fatty acid cellulose ester filmwith optical biaxiality, wherein the fatty acid cellulose ester film isprovided between the polarizing film and the liquid crystal cell.

102. The polarizing plate of item 101 above, wherein the mixed fattyacid cellulose ester film has a retardation in plane R0 of from 31 to120 nm, and a retardation in the thickness direction Rt of from 60 to300 nm, in which R0 is represented by formula (1) and Rt is representedby formula (2).

103. The polarizing plate of item 101 above, wherein the mixed fattyacid cellulose ester film has a retardation in plane R0 of from 31 to 60nm, and a retardation in the thickness direction Rt of from 90 to 200nm.

104. The polarizing plate of item 101 above, wherein the mixed fattyacid cellulose ester film has a retardation in plane R0 of from 31 to 50nm, and a retardation in the thickness direction Rt of from 110 to 150nm.

105. The polarizing plate of any one of items 1 through 4 above, whereinthe mixed fatty acid cellulose ester has an acetyl group and a propionylgroup in the ester bond.

106. The polarizing plate of any one of items 101 through 104 above,wherein the mixed fatty acid cellulose ester has an acetyl group and abutyryl group in the ester bond.

107. The polarizing plate of any one of items 101 through 106 above,wherein the mixed fatty acid cellulose ester film has a thickness offrom 30 to 110 μm.

108. The polarizing plate of any one of items 101 through 105 above anditem 107, wherein the mixed fatty acid cellulose ester film has a totalacyl substitution degree of from 1.2 to 2.8.

109. The polarizing plate of item 105 or 107 above, wherein the mixedfatty acid cellulose ester film has a total acyl substitution degree offrom 1.5 to 2.3, and a propionyl substitution degree of from 0.6 to 1.2.

110. The polarizing plate of item 108 or 109 above, wherein the mixedfatty acid cellulose ester film has an acetyl substitution degree ofless than 2.0.

111. The polarizing plate of any one of items 101 through 110 above,wherein cellulose triacetate film is provided on the surface of thepolarizing film opposite the mixed fatty acid cellulose ester film.

112. The polarizing plate of any one of items 101 through 111 above,wherein the angle formed between the orientation direction of a dichroicsubstance in the polarizing film and the direction giving a maximumrefractive index in plane of the mixed fatty acid cellulose ester filmis in the range of from 80° to 100°.

113. The polarizing plate of any one of items 101 through 112 above,wherein the angle formed between the orientation direction of a dichroicsubstance in the polarizing film and the direction giving a maximumrefractive index in plane of the mixed fatty acid cellulose ester filmis in the range of from −10° to 10°.

114. The polarizing plate of any one of items 101 through 113 above,wherein the mixed fatty acid cellulose ester film is provided on a firstsurface of the polarizing film, and the mixed fatty acid cellulose esterfilm or another cellulose ester film B is provided on a second surfaceof the polarizing film opposite the mixed fatty acid cellulose esterfilm, the stretching direction of the mixed fatty acid cellulose esterfilm being in accordance with that of the cellulose ester film B.

115. The polarizing plate of item 114 above, wherein the cellulose esterfilm B has been stretched in the transverse direction during or aftermanufacture.

116. The polarizing plate of item 114 or 115 above, wherein thecellulose ester film B has been stretched at a stretching magnificationof from 1.01 to 1.2.

117. The polarizing plate of item 14 or 15 above, wherein the celluloseester film B has been stretched at a stretching magnification of from1.05 to 1.15.

118. The polarizing plate of any one of items 114 through 117 aboveabove, wherein the cellulose ester film B is a triacetyl cellulose esterfilm.

119. The polarizing plate of any one of items 101 through 118 above,wherein when the mixed fatty acid cellulose ester film is arranged in acrossed Nicol state, and luminescent foreign materials are observed, thenumber of luminescent foreign materials with a size of from 5 to 50 μmis not more than 150 per 250 mm² of the film, and the number ofluminescent foreign materials with a size exceeding 50 μm is zero per250 mm² of the film.

120. The polarizing plate of any one of items 101 through 119 above,wherein the mixed fatty acid cellulose ester film is manufactured by aprocess comprising the steps of giving optical biaxiality to a longlength mixed fatty acid cellulose ester film, and winding the resultingfilm around a spool to obtain a long length roll film.

121. The polarizing plate of any one of items 101 through 120 above,wherein the polarizing plate is manufactured by a process comprising thesteps of giving optical biaxiality to a long length film of a mixedfatty acid cellulose ester film, adjusting the resulting film to obtaina maximum refractive index in the transverse direction of the film,winding the resulting film around a spool to obtain a long length rollfilm, providing a polarizing film containing a dichroic substance; andlaminating the polarizing film onto the roll film.

122. The polarizing plate of item 121 above, wherein the opticalbiaxiality giving step is a step in which the long length film isstretched in the transverse direction of the film when it has a residualsolvent content of not less than 10% by weight during manufacture.

123. The polarizing plate of any one of items 101 through 122 above,wherein the mixed fatty acid cellulose ester film contains 1 to 30 partsby weight of at least one plasticizer selected from the group consistingof a phosphate compound, a fatty acid ester compound, a citratecompound, and a phthalate compound based on 100 parts by weight of thecellulose ester.

124. The polarizing plate of any one of items 101 through 123 above,wherein the mixed fatty acid cellulose ester film contains 0.005 to 0.3parts by weight of particles having an average particle size of not morethan 0.1 μm, based on 100 parts by weight of the cellulose ester.

125. The polarizing plate of any one of items 101 through 124 above,wherein the mixed fatty acid cellulose ester film contains 0.8 to 3.0parts by weight of a UV absorber based on 100 parts by weight of thecellulose ester.

126. A liquid crystal display comprising a polarizing film A, apolarizing film B, and a vertical alignment mode liquid crystal cellprovided between the polarizing films A and B, one or more celluloseester films A only being provided between the polarizing film A and theliquid crystal cell, and one or more cellulose ester films B only beingprovided between the polarizing film B and the liquid crystal cell,

wherein 31 nm≦Σ R0≦120 nm, and 60 nm≦Σ Rt≦300 nm, in which Σ R0=ΣR0(A)+Σ R0(B), and Σ Rt=Σ Rt(A)+Σ Rt(B), Σ R0(A) being the sum ofretardation in plane of each of the cellulose ester films A, Σ R0(B) thesum of retardation in plane of each of the cellulose ester films B, ΣRt(A) the sum of retardation in the thickness direction of each of thecellulose ester films A, and Σ Rt(B) the sum of retardation in thethickness direction of each of the cellulose ester films B.

127. The liquid crystal display of item 126 above, having the polarizingplate of any one of items 101 through 125 on at least one surface of theliquid crystal cell, wherein the mixed fatty acid cellulose ester filmis provided between the polarizing film and the liquid crystal cell.

128. The liquid crystal display of item 119 or 120 above, wherein 60nm≦Σ R0≦120 nm, and 100 nm≦Σ Rt≦300 nm.

The present invention will be explained in detail below.

First, a cellulose ester film used in the polarizing plate of theinvention will be explained.

A conventional phase difference film with a biaxially oriented propertyused in a VA mode liquid crystal display is a film in which a liquidcrystal compound is uniformly coated on a transparent substrate andoriented or a film in which a resin such as a polycarbonate is orientedemploying a complex orientation technique. This optical film with acomplex optical anisotropy is laminated to a polarizing plate. Thepresent inventors have found a polarizing plate employing, as apolarizing plate protective film, a mixed fatty acid cellulose esterfilm (hereinafter referred to also as cellulose ester film in theinvention) with optical biaxiality in the invention, which possesses asufficient optical compensation property and maintains a stable opticalcompensation property at high temperature and high humidity. Examples ofthe polarizing plate of the invention include a so-called ellipticpolarizing plate with a phase difference function.

It has been found that a polarizing plate employing the cellulose esterfilm in the invention exhibits an excellent optical compensationproperty that provides high contrast, wide viewing angle, and minimizedcolor variation, when viewing from an oblique direction.

The cellulose ester film used in the polarizing plate of the inventionhaving an effect of increasing a viewing angle has characteristics inthat it has optical biaxiality. In the cellulose ester film, suchoptical biaxiality can be obtained by carrying out uniaxiallystretching, and biaxial stretching is not necessary. This is consideredto show that the cellulose ester film itself has a negative uniaxialproperty (nX=nY>nZ, in which nX represents refractive index in plane inthe X direction of the film, nY represents refractive index in plane inthe Y direction of the film, and nZ represents refractive index in thethickness direction of the film).

The cellulose ester film in the invention having optical biaxiality canbe used as a protective film of the polarizing plate on a liquid crystalcell side (between a dichroic substance constituting the polarizingplate and the liquid crystal cell). A support of the dichromaticsubstance opposite the liquid crystal cell (on the viewer side) is notoptically restricted, and a conventional cellulose triacetate film(hereinafter referred to also as TAC film) can be used as the protectivefilm. In this case, a TAC film, which has been stretched as a minimum inthe transverse direction by a specific stretching magnification, canprovide an excellent elliptic polarizing plate which can maintain stableoptical properties at high temperature and high humidity.

When a polarizing plate is prepared employing the cellulose ester filmin the invention, a polarizing plate having an increased viewing anglecan be prepared in the same manner as in a conventional polarizing platepreparing method, except that the cellulose ester film in the inventionis used instead of the conventional cellulose ester film used as thepolarizing plate support, which is extremely advantageous for practicaluse. The cellulose ester film in the invention is extremely preferablesupport, since in the polarizing plate preparing process, the celluloseester film saponified with an alkali can be adhered to a polarizer, andremoval of moisture from the resulting adhered material is easilycarried out. The polarizer is preferably one in which a polyvinylalcohol film doped with a dichroic substance is stretched.

The present invention can provide a viewing angle increasing polarizingplate having a viewing angle increasing property employing thepolarizing plate in the invention and a liquid crystal display employingthe viewing angle increasing polarizing plate. Further, the presentinvention improves dependency of a vertical alignment (VA) type liquidcrystal display on the viewing angle.

The polarizing plate of the invention has advantage in that theluminescent foreign materials described above are less as compared witha polarizing plate employing a cellulose ester film whose retardation inthe thickness direction Rt is increased by increasing the thickness.

Next, the optical properties of the cellulose ester film in theinvention will be explained.

In the invention, a cellulose ester film having optical biaxiality isused, and such optical biaxiality can be obtained by applying a tensionin a specific direction in the conventional cellulose estermanufacturing method including the cellulose ester dope casting step.For example, it is especially effective that stretching of the celluloseester film is carried out at the presence of a residual solvent aftercasting. Further, the cellulose ester film in the invention can beobtained by stretching a heated cellulose ester film.

The mixed fatty acid cellulose ester used for preparing the mixed fattyacid cellulose ester film in the invention is an ester of cellulose andtwo or more kinds of fatty acid. The fatty acid is preferably a lowerfatty acid. The mixed fatty acid cellulose ester has a total acylsubstitution degree of preferably more than 1.5, and more preferablyfrom more than 1.5 to not more than 2.8.

The total acyl substitution degree hereinafter referred to is a sum ofthe number of acyl groups at 2, 3, or 6 position of the glucose unit ofthe cellulose ester molecule.

In the invention, a lower fatty acid cellulose ester having a specificsubstituent, that is, an acetyl group or a propionyl group is preferablyused in obtaining an optical compensation property of more than acertain level.

The cellulose ester used in the preparation of the cellulose ester filmin the invention is preferably one having an acyl group having a carbonatom number of from 2 to 4 as a substituent, and simultaneouslysatisfying the following formulae (3) and (4):1.5≦A+B≦2.8  formula (3)0.6≦B≦1.2  formula (4)wherein A represents an acetyl substitution degree, and B represents asubstitution degree of an acyl having a carbon atom number of 3 or 4.

In the invention, the cellulose ester is more preferably onesimultaneously satisfying the following formulae (5) and (6):2.1≦A+B≦2.8  formula (5)0.6≦B≦1.2  formula (6)

In the cellulose ester, the hydrogen of the hydroxyl group of cellulosemay be evenly substituted at 2-, 3, and 6-positions with the acyl group,or may be substituted at 6-position in a high substitution rate with theacyl group.

The acyl substitution degree can be determined according toASTM-D817-96.

The cellulose ester film, in which the sum of an acetyl substitutiondegree and a substitution degree of an acyl having a carbon atom numberof 3 or 4 falls within the range described above, has a phase differencegreater to light having a longer wavelength, and can provide a good rateof water content and a water barrier property.

Particularly, the cellulose ester film having an acetyl substitutiondegree less than 2.0 is preferred in that phase difference variation atstretching is less.

The viscosity average polymerization degree (referred to also aspolymerization degree) of the cellulose ester film in the invention ispreferably from 200 to 700, and more preferably from 250 to 500, in thatan optical compensation film with excellent mechanical strength isobtained.

The viscosity average polymerization degree described above is obtainedaccording to the following method:

<<Measurement of Viscosity Average Polymerization Degree (DP)>>

Dry cellulose ester of 0.2 g is dissolved in 100 ml of a mixture solventof methylene chloride and ethanol (methylene chloride:ethanol=9:1 byweight) to obtain a cellulose ester solution. The fall time of theresulting cellulose ester solution is measured at 25° C. according to anOstwald's viscometer, and the polymerization degree is obtained from thefollowing formulae.ηrel=T/Ts  (a)[η]=(lnηrel)/C  (b)DP=[η]/Km  (c)wherein T is fall time of the cellulose ester solution, Ts is fall timeof the solvent used, C is concentration (in terms of g/liter) of thecellulose ester in the solution, and Km is 6×10⁻⁴.<<Measurement of Retardations Rt and R0>>

The cellulose ester film in the invention preferably satisfies thefollowing inequality (7), in order to obtain the effect of increasingviewing angle more efficiently.{(nx+ny)/2}−nz>0  Inequality (7)wherein nx represents refractive index in plane of the film in thedirection x giving a maximum refractive index, nx represents refractiveindex in plane of the film in the direction y perpendicular to thedirection x, and nz represents refractive index in the thicknessdirection of the film.

The cellulose ester film in the invention with optical biaxialityexhibits improved viewing angle. Viewing angle can be greatly improvedby properly controlling a retardation Rt in the thickness direction ofthe film and a retardation R0 in plane of the film. As the controlmethod of the retardation, there is, for example, a stretching methoddescribed later.

In the invention, the retardation Rt in the thickness direction of thefilm, defined by formula (2) described previously, is preferably from 60to 300 nm, more preferably from 90 to 200 nm, and most preferably from110 to 150 nm.

The retardation R0 in plane of the film is defined by formula (1)described previously. In the invention, the retardation R0 in plane ofthe film is preferably from 31 to 120 nm, more preferably from 31 to 60nm, and most preferably from 31 to 60 nm.

The three dimensional refractive index of the cellulose ester film ismeasured at 23° C. and 55% RH employing light with at a wavelength of590 nm by means of an automatic birefringence meter KOBRA-21ADH(produced by Oji Keisokukiki Co., Ltd.), to obtain refractive indicesnx, ny and nz. The retardations Rt and R0 are obtained from theresulting indices nx, ny and nz.

The cellulose ester film in the invention with optical biaxiality ispreferably a transparent support having a transmittance of not less than80%, and preferably not less than 90%. The thickness of the celluloseester film in the invention is preferably from 45 to 110 μm.

The mixed fatty acid cellulose ester used in the invention can besynthesized employing acid anhydrides and acid chlorides as theacylating agents. When the acylating agents are acid anhydrides, organicacids (for instance, acetic acid) as well as methylene chloride areemployed as the reaction solvents, and acidic catalysts such as sulfuricacid are employed as catalysts. When the acylating agents are acidchlorides, basic compounds are employed as catalysts. In the most commonsynthesis method, cellulose is subjected to esterification employingorganic acid components comprising organic acids comprising an acetylgroup and a propionyl group (for example, acetic acid and propionicacid) or those acid anhydrides (for example, acetic anhydride andpropionic anhydride), and thus cellulose esters are synthesized. Theemployed amount of the acetylating agents and propionylating agents isregulated so that the synthesized esters have the substitution degreerange as described above. The employed amount of reaction solvents ispreferably between 100 and 1,000 weight parts with respect to 100 weightparts of cellulose. The employed amount of acid catalysts is preferablybetween 0.1 and 20 weight parts, and more preferably between 0.4 and 10weight parts, with respect to 100 weight parts of cellulose.

Reaction temperature is preferably between 10 and 120° C. Further, aftercompletion of the acylation reaction, if desired, the substitutiondegree may be adjusted employing hydrolysis (saponification). Aftercompletion of the reaction, the resulting reaction mixture is separatedemploying common means such as precipitation, washed, and issubsequently dried. Thus fatty acid ester (cellulose acetate propionate)is obtained.

Employed individually or in combination as the fatty acid celluloseester of the present invention are fatty acid ester synthesizedemploying cotton linters and fatty acid ester synthesized from woodpulp. Cellulose ester synthesized employing cotton linters is preferablyemployed at a larger ratio, because it is more readily peeled from abelt or drum, and thus enhances productivity. When the content ofcellulose ester synthesized employing cotton linters is at least 60percent, the peeling properties are markedly improved. Therefore, thecontent is preferably at least 60 percent, is more preferably at least85 percent, and is most preferably 100 percent.

A cellulose ester in which cellulose is acetylated with an acetyl groupand acylated with an acyl group having a carbon number of 3 or 4 is alsocalled a mixed fatty acid ester of cellulose (a mixed fatty acidcellulose ester). Examples of the acyl group having a carbon number of 3or 4 include a propionyl group and a butyryl group. A cellulose esterfilm with a propionyl group or an n-butyryl group is preferable, and acellulose ester film with a propionyl group is more preferable, in viewof mechanical strength of the cellulose ester film or ease ofdissolution of the cellulose ester in a solvent.

Listed as solvents which dissolve the fatty acid cellulose ester of thepresent invention and form a dope may be methylene chloride, methylacetate, ethyl acetate, amyl acetate, acetone, tetrahydrofuran,1,3-dioxolan, 1,4-dioxolan, cyclohexanone, 2,2,2-trifluoroethanol,2,2,3,3-tetrafluoro-1-propanol, 1,3-difluoro-2-propanol, and the like.Incidentally, chlorine based solvents such as methylene chloride may beemployed without causing any problem from the technical aspect. It ispreferred that amount of the chlorine based solvent is not more than 50%in the solvent. Methyl acetate, ethyl acetate, acetone, and the like,cause the least environmental problem. Specifically, the content ofmethyl acetate is preferably at least 50 percent by weight with respectto the total organic solvents. Acetone is preferably employed in anamount of 5 to 30 percent by weight with respect to the total organicsolvents, together with methyl acetate, because it makes it possible todecrease the dope viscosity.

In the present invention, containing of as little as possible chlorinebased solvents means that the content of the chlorine based solvents isno more than 10 percent with respect to the total organic solvents, ismore preferably no more then 5 percent, and is most preferably 0percent.

In addition to the organic solvents described above, alcohols havingfrom 1 to 30 carbon atoms are preferably incorporated into the fattyacid cellulose ester dope of the present invention in an amount of 1 to30 percent. When alcohols are incorporated, after casting the dope ontoa support, solvents start to evaporate and the web (a dope layer formedby casting a dope on a casting support is designated as the web) isgelled and the web is strengthened. Thus it is possible to more readilypeel the web from the support. Further, it is possible to obtain effectswhich accelerate the dissolution of fatty acid cellulose ester. Listedas alcohols having from 1 to 4 carbon atoms are methanol, ethanol,n-propanol, iso-propanol, n-butanol, sec-butanol, and tert-butanol. Ofthese, ethanol is preferred, based on the stability of the resultingdope, its boiling point, drying properties, non-toxicity, and the like.

The solid portion concentration in a dope is commonly, and preferably,between 10 and 40 percent. From the viewpoint of obtaining excellentflatness of a film, the viscosity of a dope is preferably controlled tobe in the range of 100 to 500 poise.

The dope, which has been prepared as described above, is filteredemploying a filter media, defoamed, and subsequently conveyed to thenext step, employing a pump.

Plasticizers, matting agents, UV absorbers, antioxidants, dyes, and thelike may also be incorporated into said dope.

Fatty acid cellulose ester, having an acetyl group as well as apropionyl group employed in the present invention, exhibits effects of aplasticizer. As a result, sufficient film properties are obtainedwithout the addition of plasticizer, or at most addition in smallamounts. However, plasticizers may be added for other purposes. Forexample, for the purpose to enhance the moisture resistance of film,added may be alkyl phthalyl alkyl glycolates, phosphoric acid esters,carboxylic acid esters, and the like.

Listed as alkyl phthalyl alkyl glycolates are, for example, methylphthalyl methyl glycolate, ethyl phthalyl ethyl glycolate, propylphthalyl propyl glycolate, butyl phthalyl butyl glycolate, octylphthalyl octyl glycolate, methyl phthalyl ethyl glycolate, ethylphthalyl methyl glycolate, methyl phthalyl propyl glycolate, methylphthalyl butyl glycolate, ethyl phthalyl butyl glycolate, butyl phthalylmethyl glycolate, butyl phthalyl ethyl glycolate, propyl phthalyl butylglycolate, butyl phthalyl propyl glycolate, methyl phthalyl octylglycolate, ethyl phthalyl octyl glycolate, octyl phthalyl methylglycolate, octyl phthalyl ethyl glycolate, and the like.

Listed as phosphoric acid esters may be, for example, triphenylphosphate, tricresyl phosphate, cresyl diphenyl phosphate, phenyldiphenyl phosphate, octyl diphenyl phosphate, trioctyl phosphate,tributyl phosphate, and the like.

Carboxylic acid esters include, for example, phthalic acid esters andcitric acid esters. Listed as said phthalic acid esters may be dimethylphthalate, diethyl phthalate, dimethoxyethyl phthalate, dimethylphthalate, dioctyl phthalate, dibutyl phthalate, di-2-ethyl hexylphthalate, and the like.

Listed as said citric acid esters may be, for example, acetyl trimethylcitrate, acetyl triethyl citrate, and acetyl tributyl acetate.

In addition, butyl oleate, methyl acetyl recinoleate, dibutyl sebacate,triacetin, and the like are preferably employed individually or incombination.

If desired, two or more types of plasticizers may be employed incombination. Phosphoric acid ester based plasticizers are preferredbecause when employed at a ratio of no more than 50 percent, thecellulose ester film is barely subjected to hydrolysis and exhibitsexcellent durability. Further, a low content of phosphoric acid basedplasticizers is preferred. Particularly preferred is the sole use ofphthalic acid ester based or glycolic acid ester based plasticizers. Ofthese, methyl phthalyl methyl glycolate, ethyl phthalyl ethyl glycolate,propyl phthalyl propyl glycolate, butyl phthalyl butyl glycolate, andoctyl phthalyl octyl glycolate are preferred, and particularly employedis ethyl phthalyl ethyl glycolate. Alternatively, two or more types ofthese alkyl phthalyl alkyl glycolates are employed in combination. Theamount of plasticizers employed for said purpose is preferably between 1and 30 percent by weight, and is more preferably between 4 and 13percent by weight with respect to the cellulose ester. These compoundsmay be added along with cellulose ester and solvents during preparationof a cellulose ester solution, or may be added during or after thepreparation of the solution.

With the purpose to improve yellow hue of film, dyes are incorporated.Since cellulose ester film is tinted slightly yellow, dyes are preferredwhich are capable of tinting to gray as seen in common photographicsupports. Thus blue and violet dyes are preferably employed. However,being different from the photographic supports, since it is unnecessaryto minimize light piping, only a small amount of dye addition may besufficient. Specifically the content of dyes is preferably between 1 and100 ppm with respect to the cellulose ester, and is more preferablybetween 2 and 50 ppm. The cellulose ester is a little yellowish, and ablue or violet dye is preferably used. Gray may be obtained byappropriately combining a plurality of dyes.

When films are not sufficiently slippery, they are subjected to blockingwith each other, and occasionally, ease of handling is degraded. Mattingagents such as fine inorganic particles including silicon dioxide,titanium dioxide, sintered calcium silicate, hydrated calcium silicate,aluminum silicate, magnesium silicate, crosslinked polymers, and thelike are preferably incorporated into the film, based on the presentinvention.

Further, in order to decrease the haze of a film, fine particles such assilicon dioxide are preferably subjected to surface treatment employingorganic substances. Cited as preferred organic substances for saidsurface treatment are halosilanes, alkoxysilanes, silazanes, siloxanes,and the like. The matting effect increases as the average particlediameter of fine particles increases, while transparency increases assaid diameter decreases. Accordingly, the average primary particlesdiameter of fine particles is no more than 0.1 μm, preferably between 5and 50 nm, and more preferably between 7 and 14 nm. Listed as fineparticles of silicon dioxide are Aerosil 200, 200V, 300, R972, R972V,R974, R202, R812, OX50, TT600 and the like, all of which aremanufactured by Nihon Aerosil Co., ltd. of these, preferably listed areAerosil R972, R972V, R974, R202, R812, and the like. Said matting agentsare preferably blended to obtain a film haze of no more than 0.6percent, and a friction coefficient of no more than 0.5. The amount ofmatting agents, which are employed for said purpose, is preferablybetween 0.005 and 0.3 percent by weight with respect to fatty acidcellulose ester.

Liquid crystal displays are increasingly employed in the openatmosphere. Thus it is important to provide a protective film for apolarizing plate with the function to absorb ultraviolet rays. UVabsorbers are preferably incorporated into the cellulose ester film inthe invention.

Preferred as UV absorbers are those which efficiently absorb ultravioletrays having a wavelength of no longer than 370 nm from the viewpoint ofminimizing the degradation of liquid crystals and which minimally absorbvisible light having a wavelength of at least 400 nm from the viewpointof producing an excellent liquid crystal display. Specifically, thetransmittance at a wavelength of 370 nm is required to be not more than10 percent, and preferably not more than 5 percent.

UV absorbers, which are employed to achieve said purposes, preferablyhave no absorption in the visible light range. Listed as such UVabsorbers are benzotriazole based compounds, benzophenone basedcompounds, salicylic acid based compounds and the like.

Examples of such UV absorbers include2-(2′-hydroxy-5-methylphenyl)benzotriazole,2-(2′-hydroxy-3′,5′-di-t-butylphenyl)benzotriazole,2-(2′-hydroxy-3′-di-t-butyl-methylphenyl)benzotriazole,2,4-dihydroxybenzophenone, 2-hydroxy-4-methoxybenzophenone,2-hydroxy-4-n-octocybenzophenone, 4-dodecyloxy-2-hydrooxybenzophenone,2,2′,4,4′-tetrahydroxybenzophenone,2,2′-dihydroxoy-4,4′-dimethoxybenzophenone, phenyl salicylate, methylsalicylate, and the like.

In the present invention, at least one of these UV absorbers ispreferably employed, and at least two of different UV absorbers may beincorporated.

The addition methods of said UV absorbers are as follows. They may bedissolved in organic solvents such as alcohol, methylene chloride,dioxolan, and the like and the resulting solution is added to a dope.Alternatively, they may be added directly to a dope. UV absorbers suchas inorganic powders, which are not soluble in organic solvents, may bedispersed into a mixture of organic solvents and cellulose ester,employing a dissolver or a sand mill, and added to a dope.

In the present invention, the employed amount of UV absorbers iscommonly between 0.1 and 2.5 percent by weight with respect to thecellulose ester, is preferably between 0.5 and 2.0 percent by weight,and is more preferably between 0.8 and 2.0 percent by weight.

In order to enhance the heat resistance of the cellulose ester film,hindered phenol based compounds are preferably employed. The addedamount of these compounds is preferably between 1 ppm and 1.0 percent byweight with respect to the cellulose ester, and is more preferablybetween 10 and 1,000 ppm. Further, in addition to these compounds, aheat stabilizer such as a salt of an alkali earth metal, for example,calcium or magnesium may also be added.

In addition to the aforementioned compounds, further, added may beantistatic agents, flame retarders, lubricants, and the like.

The cellulose ester film in the invention is provided between twopolarizing plates, and foreign materials, which make extraordinary lightemit, result in deterioration of performance. In view of the above,occurrence of so-called luminescent foreign materials is problem.

In the invention, when luminescent foreign materials are observed in thefatty acid cellulose ester film with optical biaxiality arranged in acrossed Nicol state, it is preferred that the number of luminescentforeign materials with a size of from 5 to 50 μm is not more than 150per 250 mm² of the film, and the number of luminescent foreign materialswith a size exceeding 50 μm is zero per 250 mm² of the film.

In the invention, luminescent foreign materials, which are observed in acrossed Nicol state, imply those which are observed as bright materialswhen a cellulose ester film is placed between two polarizing platesarranged at right angles (in crossed Nicol state) and then illuminatedwith a light source from one side. Such luminescent foreign materialsare observed as bright materials in a dark visual field in crossed Nicolstate, and therefore, it is possible to readily determine the size aswell as the number of luminescent foreign materials. The cellulose esterfilm in the invention can decrease its thickness compared with a phasedifference plate with a high Rt, and can reduce the number of foreignmaterials observed as bright materials.

Regarding luminescent foreign materials observed in a crossed Nicolstate, the number of luminescent foreign materials with a size of from 5to 50 μm is not more than 150, preferably not more than 50, and morepreferably from 0 to 10, per 250 mm² of the film, and the number ofluminescent foreign materials with a size exceeding 50 μm is zero per250 mm² of the film. Many luminescent foreign materials have an adverseeffect on image quality of a liquid crystal display.

Next, a manufacturing method of the cellulose ester film in theinvention will be explained. The cellulose ester film manufacturingmethod is preferably a solution cast film manufacturing methodcomprising the steps of casting a cellulose ester dope onto a support toform a web, peeling the web from the support, applying a tension to thepeeled web, and drying it while transporting on a drying zone. Thesolution cast film manufacturing method will be explained below.

(1) Dissolution Step

The dissolution step is one in which cellulose ester flakes aredissolved, while stirring, in organic solvents mainly comprised of goodsolvents for said flakes, employing a dissolution vessel, and thereby acellulose ester solution (hereinafter referred to as a dope) isprepared. In order to carry out said dissolution, there are variousmethods such as a method in which dissolution is carried out at a normalatmospheric pressure, a method in which dissolution is carried out at atemperature lower than the boiling point of the primary solvent, amethod in which dissolution is carried out at a temperature higher thanthe boiling point of the main solvent under an increase of pressure, acooling dissolution method in which dissolution is carried out at alowering temperature, as described in J. M. G. Cowie et al., Makromolhem., volume 143, page 105 (1971), and Japanese Patent Publication Opento Public Inspection Nos. 9-95544 and 9-95557, and others, a method inwhich dissolution is carried out at a high pressure, and the like. Theresultant dope is filtered employing filter materials, is then defoamed,and is subsequently pumped to the next step.

(2) Casting Step

The casting step is one in which a dope is conveyed to a pressure diethrough a pressure type metering gear pump, and cast from said pressuredie on a support (hereinafter referred to also as a support) for castingsuch as a moving endless metal belt or a rotating metal drum at acasting position. The surface of the support for casting is specular. Asother casting steps, there is a doctor blade method adjusting the dopethickness of the cast dope with a doctor blade or a reverse rollermethod adjusting the dope thickness of the cast dope with a reverseroller coater rotating reversely. A pressure die is preferred in whichthe slit shape at the mouth piece portion can be regulated and the filmthickness is readily regulated to be uniform. Examples of the pressuredie include a coat hanger die, a “T” die, and the like, and any of theseis preferably employed. In order to increase the casting speed, two ormore pressure dies may be provided on the metal support and dopesdivided into two or more may be simultaneously cast on the metalsupport. A laminated cellulose ester film can be also prepared by aco-casting method.

(3) Solvent Evaporation Step

The solvent evaporation step is one in which a web (a film formed aftera dope is cast on a support for casting is called a web) is heated on asupport for casting and solvents are evaporated. In order to evaporatesolvents, methods include a method in which air is blown from the webside, and/or a method in which heating is carried out from the reversesurface of the support employing liquid, and a method in which heatingis carried out from the surface as well as the revere surface employingheat radiation. Of these, the reverse surface liquid heating method ispreferred due to high drying efficiency. Further, these methods arepreferably combined.

(4) Peeling Step

The peeling step is one in which a web, which has been subjected toevaporation of solvents on the support, is peeled at the peelingposition. The peeled web is conveyed to the subsequent step. When theresidual solvent amount (represented by the formula described below) istoo large, it may be difficult to peel the web. On the contrary, whenpeeling is carried out after fully drying the web on the support, a partof the web may peel prior to the peeling position.

Listed as a method to increase the film forming speed is a gel castingmethod (in which peeling can be carried out even though the amount ofresidual solvents is relatively great). The gel casting methods includea method in which poor solvents with respect to the cellulose ester areadded to a dope and gelling is carried out after casting said dope, andalso a method in which gelling is carried out by decreasing thetemperature of a support, and the like. There is further a method inwhich metal salts are added to the cellulose ester dope. Bystrengthening the web through gelling the dope on the support, it ispossible to carry out earlier peeling and to increase the casting speed.When the peeling is carried out at the time when the residual solventamount is still relatively great, the web may be too soft, in whichduring peeling, the flatness of the web tends to be degraded, andwrinkles and longitudinal streaks due to the peeling tension tend to beformed. Accordingly, the residual solvent amount at the peeling isdetermined so that productivity and quality are balanced.

(3) Drying Step

The drying step is a step, which dries the web employing a dryingapparatus in which said web is alternatively transported throughstaggered rollers and/or a tenter apparatus in which said web istransported while holding both edges of the web employing clips. Anordinal common drying method is one in which both surfaces of the webare heated by heated air flow. Instead of the air flow, employed is amethod in which heating is carried out employing microwaves. Too rapiddrying tends to degrade the flatness of the finished film. A hightemperature drying is preferably applied to the web having a residualsolvent content of not more than 8% by weight. During the entire dryingstep, drying temperature is ordinarily from 40 to 250° C., andpreferably 70 to 180° C. Drying temperature, drying time, and air volumefor drying vary depending on employed solvents. Thus, drying conditionsmay be properly selected depending on types of employed solvents andtheir combination.

In the drying step after peeling the web from the support, the web tendsto shrink in the transverse direction due to evaporation of the organicsolvent. When the web is dried rapidly at a higher temperature, thetendency of the web shrinkage is stronger. Drying of the web while theshrinkage is minimized as far as possible is preferable in improving aflatness of the finished cellulose ester film. In view of the above, amethod disclosed in Japanese Patent O.P.I. Publication No. 62-46625 ispreferably used which comprises drying the web while holding the bothedges in the transverse direction of the web by clips or pins tomaintain the web width (a tenter method).

(6) Winding Step

This step is one in which after the residual solvent content of the webis not more than 2% by weight, the resulting cellulose ester film iswound around a spool. The cellulose ester film having a residual solventcontent of not more than 0.4% by weight provides good dimensionalstability. Any of conventional winding methods can be used, and examplesof the winding methods include a constant torque method, a constanttension method, a taper tension method, and a method programmed so as tohave a constant inside stress.

The thickness of the fatty acid cellulose ester film can be adjusted bycontrolling a dope concentration, a dope amount supplied by a pump, aslit width of the mouth piece portion of a die, an extrusion pressure ofa die, or a moving speed of a support for casting. It is preferred thatthe thickness of the cellulose ester film is uniformly regulated byfeeding back a thickness information detected by a thickness detector tothe devices described above through a system programmed in advance forinformation to be fed back to them.

In the step from the casting to the drying in the solution cast filmmanufacture process, drying of the web in the drying apparatus may becarried out at an air atmosphere or at an atmosphere of an inert gassuch as a nitrogen gas, a carbon dioxide gas or an argon gas. It is notneedless to say that the explosion limits of the vaporized organicsolvents in the drying apparatus should be always considered.

The cellulose ester film in the invention having optical biaxiality canbe prepared according to any method for obtaining an orientation showingoptical biaxiality (showing the relation nx>ny>Nz), but one of themethods, which are effective, is a stretching method.

In the preparation of the cellulose ester film in the invention,stretching can be carried out at not so high temperature controlling theresidual solvent in the film as described later, but can be also carriedout at high temperature when the residual solvent is not controlled.When stretching of the cellulose ester film is carried out at hightemperature, the stretching temperature is not less than the glasstransition point of the cellulose ester, some of the plasticizersdescribed above reduce the effect in stretching, and does not provide asufficient stretching property. Therefore, a plasticizer capable ofproviding a sufficient stretching property at high temperature isnecessary, and it has been found that a plasticizer havingnon-volatility can be used as such a plasticizer. The plasticizer havingnon-volatility herein referred to is a compound with a vapor pressure at200° C. of not more than 1330 Pa, which has an extremely low vaporpressure and an extremely low volatility. The vapor pressure at 200° C.is preferably not more than 665 Pa, and more preferably not more than133 Pa. For example, arylene bis(diarylphosphate) ester is preferred.Besides, tricresyl phosphate (vapor pressure: 38.6 Pa, 200° C.), ortris(2-ethylhexyl) trimellitate (vapor pressure: 66.5 Pa, 200° C.) arepreferably used. Further, non-volatile phosphates, disclosed I JapanesePatent Publication No. 501040, are preferably used. High molecularweight plasticizers such as oligomers and polymers including acryl resinand polyvinyl acetate are preferably used. The plasticizer content ofthe cellulose ester film is preferably 0.1 to 30% by weight, and morepreferably 0.5 to 14% by weight, based on the cellulose ester. Employingthe plasticizer, the stretching property at high temperature ofcellulose ester film can be improved, and cellulose ester film withexcellent surface quality or excellent flatness can be efficientlymanufactured.

As one of the preferred methods for giving optical biaxiality to thecellulose ester film in the invention, there is a method in which acellulose ester web containing a residual solvent is stretched. Such astretching method will be explained below.

In the manufacture process of the cellulose ester film in the invention,the cellulose ester dope is cast on a support for casting to form a web(web film), and the web is peeled from the support. The peeled web ispreferably stretched in at least one direction by a factor of from 1.0to 4.0, while the residual solvent content of the web is not more than100% by weight, and preferably 10 to 100% by weight.

The residual solvent content of the web is represented by the followingformula:Residual solvent content (% by weight)={(M−N)/N}×100wherein M represents weight of the web to be measured, and N representsweight after the web has been dried at 110° C. for three hours.

When the residual solvent content of the web is too high, the effectsdue to stretching of the web cannot be obtained. When the residualsolvent content is too low, it is difficult to stretch the web, whichmay result in web rupture. The residual solvent content of the web ismore preferably from 10 to 500% by weight, and still more preferably 15to 40% by weight.

When the stretching magnification is too low, sufficient phasedifference cannot be obtained, while when the stretching magnificationis too high, it is difficult to stretch the web, which may result in webrupture. The stretching magnification is more preferably from 1.0 to3.5.

A web formed after casting the dope of the cellulose ester in theinvention, when the web has a specific residual solvent content, can bestretched without being heated to high temperature. It is preferred thatstretching and drying are simultaneously carried out, since the steps ofthe film manufacture are reduced. When the web temperature is too high,plasticizer contained in the web is volatilized. Therefore, the webtemperature is preferably from room temperature (15° C.) to 180° C.

Stretching the web in two axis directions perpendicular to each other isan efficient method to adjust refractive indices nx, ny, and nz of thefilm to the range defined in the invention.

Variation in thickness of the film can be reduced by stretching the webin two axis directions perpendicular to each other. The cellulose esterfilm with great variation in the thickness causes unevenness of phasedifference, and when such a film is used in an optical compensationfilm, there occurs a problem such as coloring. Variation in thickness ofthe cellulose ester film is within the range of preferably ±3%, and morepreferably ±1%. In view of the above, stretching the web in two axisdirections perpendicular to each other is effective, and it is preferredthat the stretching magnification in one axis direction and thestretching magnification in the other direction are from 0.8 to 4.0 and0.4 to 1.2, respectively.

A method of stretching the web is not specifically limited. There are,for example, a method of stretching the web in the mechanical directionemploying plural transporting rollers having different peripheralspeeds, a method of fixing both ends of the web with clips or pins andstretching the web in the mechanical direction broadening the intervalsof the clips or pins in the transporting direction, and a method ofstretching the web in the mechanical and transverse directionssimultaneously broadening the intervals of the clips or pins in thetransporting and transverse directions. These methods may be used incombination. When so-called a tenter is used, and the clips are drivenaccording to a linear drive method, a smooth stretching can be realized,reducing danger such as web rupture.

It is preferred in view of dimensional stability that the finallyfinished cellulose ester film has a residual solvent content of not morethan 2% by weight, and preferably not more than 0.4% by weight.

It is preferred in the invention that the various conditions describedabove are controlled at the cast film manufacture to give a maximumrefractive index in the transverse direction of the cellulose esterfilm.

As described above, the cellulose ester film with optical biaxiality inthe invention satisfies inequality nx>ny>nz. In the invention, “amaximum refractive index in the transverse direction” described aboveimplies that the direction giving nx is approximately in accordance withthe transverse direction.

Herein, that one of the two directions is approximately in accordancewith the other implies that one of the two axis directions isapproximately in parallel with the other. Further, that one of the twoaxis directions is approximately in parallel with the other implies thatthe angle formed between the two axis directions is within the range of±10°, preferably ±3°, and more preferably ±1°.

In the polarizing plate of the invention, the cellulose ester film withoptical biaxiality is preferably laminated onto a polarizing filmcontaining a dichroic substance, so that the transmission axis of thepolarizing film is approximately in parallel with the stretchingdirection in the transverse direction of the film at the filmmanufacture process.

In preparation of the polarizing plate of the invention, a celluloseester film with optical biaxiality is laminated onto a polarizing filmcontaining a dichroic substance. As the cellulose ester film, a longlength cellulose ester roll film is preferably used in view ofproduction efficiency. In the invention, the long length cellulose esterroll film has a length of not less than 500 m, preferably not less than1000 m, and still more preferably from 1000 to 4000 m.

The polarizing plate of the invention and the liquid crystal display ofthe invention employing the polarizing plate will be explained below.

A conventional polarizing film can be used as the polarizing film usedin the polarizing plate of the invention. For example, a polarizingfilm, in which a film of a hydrophilic polymer such as polyvinyl alcoholis treated with a dichromatic dye such as iodine, and stretched, or apolarizing film, in which a plastic film such as polyvinyl chloride issubjected to orientation treatment, can be used. The thus obtainedpolarizing film is laminated with a cellulose ester film.

In the invention, it is essential that the cellulose ester film in theinvention be provided on at least one side of the polarizing film of thepolarizing plate. Although cellulose triacetate (TAC) film used in thesupport of a conventional polarizing plate may be provided on the otherside of the polarizing film of the polarizing plate, all the celluloseester films used in the polarizing plate are preferably the celluloseester film in the invention in view of similarity in physical propertiesof both surfaces of the polarizing plate, whereby the effect of theinvention is obtained most efficiently.

A polarizing plate, which comprises the cellulose ester film in theinvention, and provided thereon, a polarizing film and a secondcellulose acetate film having been stretched in the transverse directionin that order, provides a polarizing plate with phase differencefunction which provides reduced dimensional change (shape change) undervarious temperature and humidity circumstances, and maintains excellentoptical properties. The stretching in the transverse direction may becarried out during or after the film manufacture, however, it ispreferred in view of productivity or uniformity of stretching that thestretching is continuously carried out during the film manufacture. Thestretching magnification is preferably from 1.01 to 1.2, more preferablyfrom 1.03 to 1.15, and most preferably from 1.05 to 1.10. The secondcellulose acetate film is preferably cellulose triacetate (TAC) film.

In the above polarizing plate, the ratio A/B of stretching magnificationA of the cellulose ester film in the invention provided on one side ofthe polarizing film to stretching magnification B of the cellulosetriacetate film provided on the other side of the polarizing film ispreferably from 1000 to 0.001, more preferably from 200 to 0.005, andstill more preferably from 100 to 0.01.

The polarizing plate is preferably a polarizing plate in which thecasting direction during manufacture of the cellulose ester film in theinvention is approximately parallel with the stretching direction of thepolarizing film. The thus obtained polarizing plate is provided on bothsides of a liquid crystal cell, and preferably has the followingconstitution.

The polarizing plate of the invention has the constitution shown in, forexample, FIG. 1. In FIG. 1, a liquid crystal display 9 is comprised of acrystal cell 7 and two polarizing plates 6 a and 6 b.

The polarizing plate 6 a is comprised of two cellulose ester films 1 aand one polarizing film 2 a, and the polarizing plate 6 b is comprisedof two cellulose ester films 1 b and one polarizing film 2 b.

In polarizing plate 6 a, an arrow 3 a represents the casting directionduring manufacture of the cellulose ester film 1 a, and in polarizingplate 6 b, an arrow 3 b represents the casting direction duringmanufacture of the cellulose ester film 1 b. An arrow 4 a represents thestretching direction of the polarizing film 2 a, and an arrow 4 brepresents the stretching direction of the polarizing film 2 b. A VAmode liquid crystal display with the simple constitution described abovecan be obtained in which visibility in the oblique direction is greatlyimproved.

EXAMPLES

The invention will be detailed according to the following examples, butis not limited thereto.

Example 1

<<Preparation of Cellulose Ester Film>>

(Preparation of Cellulose Ester Film 1)

Cellulose acetate propionate (with an acetyl substitution degree of 2.00and a propionyl substitution degree of 0.80 and a viscosity averagepolymerization degree of 350) of 100 parts by weight, 5 parts by weightof ethylphthalylethyl glycolate, 3 parts by weight of triphenylphosphate, 290 parts by weight of methylene chloride, and 60 parts byweight of ethanol were placed into a tightly sealed vessel, andgradually heated to 45° C. in 60 minutes while slowly stirred. Pressurein the vessel was 121 kPa. The resulting solution was filtered employingAzumi filter paper No. 244, produced by Azumi Roshi Co., Ltd., andallowed to stand for 24 hours to remove foams in the solution. Thus, acellulose ester solution was prepared.

Five parts by weight of the cellulose acetate propionate describedabove, 6 parts by weight of TINUVIN 326 (produced by Ciba Specialty Co.,Ltd.), 4 parts by weight of TINUVIN 109 (produced by Ciba Specialty Co.,Ltd.), and 5 parts by weight of TINUVIN 171 (produced by Ciba SpecialtyCo., Ltd.) were dissolved in a solvent of 94 parts by weight ofmethylene chloride and 8 parts by weight of ethanol. Thus, a UV absorbersolution was obtained.

One hundred parts by weight of the cellulose ester solution were mixedwith 2 parts by weight of the UV absorber solution and the mixture wasstirred with a static mixer to prepare a dope. The dope of 30° C. wascast at a width of 1.6 m on a stainless steel belt support from a diecoater, dried for 1 minute on the stainless steel whose the rear sidewas brought into contact with 25° C. water, and maintained for 15seconds on the stainless steel cooled whose rear side was furtherbrought into contact with a 15° C. chilled water to form a web. Then,the web was peeled from the support. The residual solvent content at thepeeling of the web was 100% by weight. Subsequently, employing a tenterfor stretching, both edges of the peeled web were held with clips andthe distance between the clips was changed by applying a tension only inthe transverse direction of the web at 120° C., so that the web wasstretched in the transverse direction at a stretching magnification of1.65. The stretched web was dried at 120° C. for 10 minutes whiletransported on the rollers to prepare cellulose ester film 1 with athickness of 110 μm.

The resulting cellulose ester film 1 was wound around a core of a glassfiber reinforced resin with a diameter of 200 mm according to a tapertension method to form a film roll having a width of 1 m and a length of1000 m. In this step, a 250° C. embossing ring was applied to the edgesof the film for embossing treatment in order to prevent film adherencein the roll film. Thus, roll film was obtained.

Three cellulose ester film samples were taken from the central portionin the transverse direction of the resulting roll film, and refractiveindex nx in the delayed phase axis direction, refractive index ny in theadvanced phase axis direction, and refractive index nz in the thicknessdirection were measured. Retardation in plane R0 and retardation in thethickness direction Rt of the samples were calculated from themeasurements employing formulae (1) and (2) described above. In thecentral portion, R0 was 88 nm, and Rt was 205 nm. The direction of thedelayed phase axis of the samples was within ±0.7° with respect to thetransverse direction of the film.

Measurement of the three dimensional birefringence of the celluloseester film was made at 23° C. and 55% RH employing light with at awavelength of 590 nm by means of an automatic birefringence meterKOBRA-21ADH (produced by Oji Keisokukiki Co., Ltd.), and then,refractive indices nx, ny and nx were determined. Further, the rate ofwater content of the sample was determined employing a measuring methodof rate of water content described later. The rate of water content ofthe sample was 1.8%.

(Preparation of Cellulose Ester Film 2)

Cellulose acetate propionate (with an acetyl substitution degree of 1.90and a propionyl substitution degree of 0.75 and a viscosity averagepolymerization degree of 350) of 100 parts by weight, 2 parts by weightof ethylphthalylethyl glycolate, 8.5 parts by weight of triphenylphosphate, 290 parts by weight of methylene chloride, and 60 parts byweight of ethanol were placed into a tightly sealed vessel, andgradually heated to 45° C. in 60 minutes while slowly stirred. Pressurein the vessel was 121 kPa. The resulting solution was filtered employingAzumi filter paper No. 244, produced by Azumi Roshi Co., Ltd., andallowed to stand for 24 hours to remove foams in the solution. Thus, acellulose ester solution was prepared.

Five parts by weight of the cellulose acetate propionate describedabove, 6 parts by weight of TINUVIN 326 (produced by Ciba Specialty Co.,Ltd.), 4 parts by weight of TINUVIN 109 (produced by Ciba Specialty Co.,Ltd.), and 5 parts by weight of TINUVIN 171 (produced by Ciba SpecialtyCo., Ltd.) were dissolved in a solvent of 94 parts by weight ofmethylene chloride and 8 parts by weight of ethanol. Thus, a UV absorbersolution was obtained.

One hundred parts by weight of the cellulose ester solution were mixedwith 2 parts by weight of the UV absorber solution and the mixture wasstirred with a static mixer to prepare a dope. The dope of 30° C. wascast at a width of 1.6 m on a stainless steel belt support from a diecoater, dried for 1 minute on the stainless steel whose the rear sidewas brought into contact with 25° C. water, and maintained for 15seconds on the stainless steel cooled whose rear side was furtherbrought into contact with a 15° C. chilled water to form a web. Then,the web was peeled from the support. The residual solvent content at thepeeling of the web was 90% by weight. Subsequently, employing a tenterfor stretching, both edges of the peeled web were held with clips andthe distance between the clips was changed by applying a tension only inthe transverse direction of the web at 120° C., so that the web wasstretched in the transverse direction at a stretching magnification of1.40. The stretched web was dried at 125° C. for 10 minutes whiletransported on the rollers to prepare cellulose ester film 2 with athickness of 70 μm.

The resulting cellulose ester film 2 was wound around a core of a glassfiber reinforced resin with a diameter of 200 mm according to a tapertension method to form a film roll having a width of 1 m and a length of1000 m. In this step, a 250° C. embossing ring was applied to the edgesof the film for embossing treatment in order to prevent film adherencein the roll film. Thus, roll film was obtained.

A cellulose ester film sample was taken from the central portion in thetransverse direction of the resulting roll film. Refractive indices nx,ny, and nz of the sample were measured in the same manner as above, andretardation in plane R0 and retardation in the thickness direction Rt ofthe sample were calculated. In the central portion of the sample, R0 was47 nm, and Rt was 132 nm. Further, the rate of water content of thesample was 1.8%.

(Preparation of Cellulose Ester Film 3)

Cellulose acetate propionate (with an acetyl substitution degree of 1.60and a propionyl substitution degree of 1.20 and a viscosity averagepolymerization degree of 400) of 100 parts by weight, 5 parts by weightof ethylphthalylethyl glycolate, 3 parts by weight of triphenylphosphate, 290 parts by weight of methylene chloride, and 60 parts byweight of ethanol were placed into a tightly sealed vessel, andgradually heated to 45° C. in 60 minutes while slowly stirred. Pressurein the vessel was 121 kPa. The resulting solution was filtered employingAzumi filter paper No. 244, produced by Azumi Roshi Co., Ltd., andallowed to stand for 24 hours to remove foams in the solution. Thus, acellulose ester solution was prepared. Five parts by weight of thecellulose acetate propionate described above, 6 parts by weight ofTINUVIN 326 (produced by Ciba Specialty Co., Ltd.), 4 parts by weight ofTINUVIN 109 (produced by Ciba Specialty Co., Ltd.), 5 parts by weight ofTINUVIN 171 (produced by Ciba Specialty Co., Ltd.) and 1 part by weightof AEROSIL R972V (produced by NIPPON AEROSIL Co., Ltd.) were dissolvedin a solvent of 94 parts by weight of methylene chloride and 8 parts byweight of ethanol. Thus, a UV absorber solution was obtained. R972V wasadded as a dispersion in which R972V had been dispersed in the aboveethanol.

One hundred parts by weight of the cellulose ester solution were mixedwith 2 parts by weight of the UV absorber solution and the mixture wasstirred with a static mixer to prepare a dope. The dope of 30° C. wascast at a width of 1.6 m on a stainless steel belt support from a diecoater, dried for 1 minute on the stainless steel whose the rear sidewas brought into contact with 25° C. water, and maintained for 15seconds on the stainless steel cooled whose rear side was furtherbrought into contact with a 15° C. chilled water to form a web. Then,the web was peeled from the support. The residual solvent content at thepeeling of the web was 90% by weight. Subsequently, employing a tenterfor stretching, both edges of the peeled web were held with clips andthe distance between the clips was changed by applying a tension only inthe transverse direction of the web at 100° C., so that the web wasstretched in the transverse direction at a stretching magnification of1.5. The stretched web was dried at 120° C. for 10 minutes whiletransported on the rollers to prepare cellulose ester film 3 with athickness of 90 μm.

The resulting cellulose ester film 3 was wound around a core of a glassfiber reinforced resin with a diameter of 200 mm according to a tapertension method to form a film roll having a width of 1 m and a length of1000 m. In this step, a 250° C. embossing ring was applied to the edgesof the film for embossing treatment in order to prevent film adherencein the roll film. Thus, roll film was obtained.

Three cellulose ester film samples were taken from the central portionin the transverse direction of the resulting roll film. Refractiveindices nx, ny, and nz of the samples were measured in the same manneras above, and retardation in plane R0 and retardation in the thicknessdirection Rt of the sample were calculated. In the central portion ofthe samples, R0 was 85 nm, and Rt was 210 nm. The direction of thedelayed phase axis of the samples was within ±0.4° with respect to thetransverse direction of the film. Further, the rate of water content ofthe samples was 2.1%.

(Preparation of Cellulose Ester Film 4)

Cellulose acetate propionate (with an acetyl substitution degree of 2.00and a propionyl substitution degree of 0.80 and a viscosity averagepolymerization degree of 350) of 100 parts by weight, 5 parts by weightof ethylphthalylethyl glycolate, 3 parts by weight of triphenylphosphate, 175 parts by weight of methyl acetate, and 75 parts by weightof ethanol were placed into a tightly sealed vessel, and graduallyheated to 65° C. in 60 minutes while slowly stirred. Pressure in thevessel was 121 kPa. The resulting solution was filtered employing Azumifilter paper No. 244, produced by Azumi Roshi Co., Ltd., and allowed tostand for 24 hours to remove foams in the solution. Thus, a celluloseester solution was prepared. Five parts by weight of the celluloseacetate propionate described above, 6 parts by weight of TINUVIN 326(produced by Ciba Specialty Co., Ltd.), 4 parts by weight of TINUVIN 109(produced by Ciba Specialty Co., Ltd.), and 5 parts by weight of TINUVIN171 (produced by Ciba Specialty Co., Ltd.) were dissolved in a solventof 94 parts by weight of methyl acetate and 8 parts by weight ofethanol. Thus, a UV absorber solution was obtained. One hundred parts byweight of the cellulose ester solution were mixed with 2 parts by weightof the UV absorber solution and the mixture was stirred with a staticmixer to prepare a dope. The dope of 50° C. was cast at a width of 1.6 mon a stainless steel belt support from a die coater, dried for 1 minuteon the stainless steel whose the rear side was brought into contact with55° C. water, and maintained for 15 seconds on the stainless steelcooled whose rear side was further brought into contact with a 15° C.chilled water to form a web. Then, the web was peeled from the support.The residual solvent content at the peeling of the web was 80% byweight. Subsequently, employing a tenter for stretching, both edges ofthe peeled web were held with clips in a tenter for simultaneous biaxialstretching and the web was stretched at 120° C. by simultaneouslyapplying a tension both in the transverse direction and in themechanical direction, so that the web was stretched in the transversedirection at a stretching magnification of 1.55, and stretched in themechanical direction at a stretching magnification of 1.05. Thestretched web was dried at 130° C. for 10 minutes while transported onthe rollers to prepare cellulose ester film 4 with a thickness of 120μm.

The resulting cellulose ester film 3 was wound around a core of a glassfiber reinforced resin with a diameter of 200 mm according to a tapertension method to form a film roll having a width of 1 m and a length of1000 m. In this step, a 250° C. embossing ring was applied to the edgesof the film for embossing treatment in order to prevent film adherencein the roll film. Thus, roll film was obtained.

Three cellulose ester film samples were taken from the central portionin the transverse direction of the resulting roll film. Refractiveindices nx, ny, and nz of the samples were measured in the same manneras above, and retardation in plane R0 and retardation in the thicknessdirection Rt of the sample were calculated. In the central portion ofthe samples, R0 was 93 nm, and Rt was 227 nm. The direction of thedelayed phase axis of the samples was within ±0.9° with respect to thetransverse direction of the film. Further, the rate of water content ofthe samples was 1.6%.

(Preparation of Cellulose Ester Film 5) Cellulose ester film 5 with athickness of 75 μm and its roll film were prepared in the same manner asin cellulose ester film 1, except that cellulose acetate butyrate (withan acetyl substitution degree of 1.90 and a butyryl substitution degreeof 0.75 and a viscosity average polymerization degree of 300) was usedinstead of cellulose acetate butyrate.

Three cellulose ester film samples were taken from the central portionin the transverse direction of the resulting roll film. Refractiveindices nx, ny, and nz of the samples were measured in the same manneras above, and retardation in plane R0 and retardation in the thicknessdirection Rt of the sample were calculated. In the central portion ofthe samples, R0 was 44 nm, and Rt was 136 nm. The direction of thedelayed phase axis of the samples was within ±1.0° with respect to thetransverse direction of the film. Further, the rate of water content ofthe samples was 1.3%.

(Preparation of Cellulose Ester Film 6)

Cellulose ester film 6 with a thickness of 75 μm and its roll film wereprepared in the same manner as in cellulose ester film 1, except thatthe stretching magnification was 1.55 and the film thickness was 48 μm.

Three cellulose ester film sample were taken from the central portion inthe transverse direction of the resulting roll film. Refractive indicesnx, ny, and nz of the samples were measured in the same manner as above,and retardation in plane R0 and retardation in the thickness directionRt of the sample were calculated. In the central portion of the samples,R0 was 43 nm, and Rt was 126 nm. The direction of the delayed phase axisof the samples was within ±0.7° with respect to the transverse directionof the film. Further, the rate of water content of the samples was 1.1%.

(Preparation of Cellulose Ester Film 7)

Cellulose acetate propionate (with an acetyl substitution degree of 2.30and a propionyl substitution degree of 0.5 and a viscosity averagepolymerization degree of 300) of 100 parts by weight, 5 parts by weightof ethylphthalylethyl glycolate, 3 parts by weight of triphenylphosphate, 290 parts by weight of methylene chloride, and 60 parts byweight of ethanol were placed into a tightly sealed vessel, andgradually heated to 45° C. in 60 minutes while slowly stirred. Pressurein the vessel was 121 kPa. The resulting solution was filtered employingAzumi filter paper No. 244, produced by Azumi Roshi Co., Ltd., andallowed to stand for 24 hours to remove foams in the solution. Thus, acellulose ester solution was prepared.

Three parts by weight of the cellulose acetate propionate describedabove, 3 parts by weight of TINUVIN 326 (produced by Ciba Specialty Co.,Ltd.), 4 parts by weight of TINUVIN 109 (produced by Ciba Specialty Co.,Ltd.), and 5 parts by weight of TINUVIN 171 (produced by Ciba SpecialtyCo., Ltd.) were dissolved in a solvent of 90 parts by weight ofmethylene chloride and 10 parts by weight of ethanol. Thus, a UVabsorber solution was obtained.

One hundred parts by weight of the cellulose ester solution were mixedwith 2 parts by weight of the UV absorber solution and the mixture wasstirred with a static mixer to prepare a dope. The dope of 30° C. wascast on a stainless steel belt support from a die coater, dried for 1minute on the stainless steel whose the rear side was brought intocontact with 35° C. water, and maintained for 15 seconds on thestainless steel cooled whose rear side was further brought into contactwith a 15° C. chilled water to form a web. Then, the web was peeled fromthe support. The residual solvent content at the peeling of the web was70% by weight.

Subsequently, the peeled web was transported through the rollers in a120° C. oven, where the peripheral speed of the rollers at the outlet ofthe oven was 1.7 times the peripheral speed of the rollers at the inletof the oven so that the web was stretched in the mechanical direction ata stretching magnification of 1.7. Immediately after that, the web wascooled to 60° C. Then, the resulting web was dried at 130° C. for 5minutes in a tenter while both edges of the web were held with clips,the distance between the clips being fixed, to prepare cellulose esterfilm 7 with a thickness of 115 μm.

The resulting cellulose ester film 7 was wound around a core of a glassfiber reinforced resin with a diameter of 200 mm according to a tapertension method to form a film roll having a width of 1 m and a length of1000 m. In this step, a 270° C. embossing ring was pressed onto theedges of the film for embossing treatment to give a height of 10 μm inorder to prevent film adherence in the roll film. Thus, roll film wasobtained.

Three cellulose ester film samples were taken from the central portionin the transverse direction of the resulting roll film. Refractiveindices nx, ny, and nz of the samples were measured in the same manneras above, and retardation in plane R0 and retardation in the thicknessdirection Rt of the sample were calculated. In the central portion ofthe samples, R0 was 102 nm, and Rt was 163 nm. The direction of thedelayed phase axis of the samples were within ±0.5° with respect to thetransverse direction of the film. Further, the rate of water content ofthe samples was 2.0%.

<<Preparation of Comparative Film 1 (TAC Film 1)>>

A conventional cellulose triacetate film (TAC film 1), which has beenused as a substrate for a polarizing plate, was prepared according tothe following procedures.

Cellulose triacetate (with an acetyl substitution degree of 2.92 and aviscosity average polymerization degree of 300) of 100 parts by weight,2 parts by weight of ethylphthalylethyl glycolate, 10 parts by weight oftriphenyl phosphate, 350 parts by weight of methylene chloride, and 50parts by weight of ethanol were placed into a tightly sealed vessel, andgradually heated to 45° C. in 60 minutes while slowly stirred. Pressurein the vessel was 121 kPa. The resulting solution was filtered employingAzumi filter paper No. 244, produced by Azumi Roshi Co., Ltd., andallowed to stand for 24 hours to remove foams in the solution. Thus, acellulose triacetate solution was prepared.

Five parts by weight of the cellulose triacetate described above, 3parts by weight of TINUVIN 326 (produced by Ciba Specialty Co., Ltd.), 7parts by weight of TINUVIN 109 (produced by Ciba Specialty Co., Ltd.), 5parts by weight of TINUVIN 171 (produced by Ciba Specialty Co., Ltd.)and 1 part by weight of AEROSIL 200V (produced by NIPPON AEROSIL Co.,Ltd.) were dissolved in a solvent of 90 parts by weight of methylenechloride and 10 parts by weight of ethanol. Thus, a UW absorber solutionwas obtained. One hundred parts by weight of the cellulose triacetatesolution were mixed with 2 parts by weight of the UV absorber solutionand the mixture was stirred with a static mixer to prepare a dope. Thedope of 30° C. was cast on a stainless steel belt support from a diecoater, dried for 1 minute on the stainless steel whose the rear sidewas brought into contact with 30° C. water, and maintained for 15seconds on the stainless steel cooled whose rear side was furtherbrought into contact with a 15° C. chilled water to form a web. Then,the web was peeled from the support.

The residual solvent content at the peeling of the web was 70% byweight. Subsequently, both edges of the peeled web were held with clipsand the web was dried at 130° C. for 10 minutes while the distancebetween the clips was fixed to prepare cellulose triacetate film 1 (TACfilm 1) with a thickness of 80 μm.

The resulting TAC film 1 was wound around a core of a glass fiberreinforced resin with a diameter of 200 mm according to a taper tensionmethod to form a film roll having a width of 1 m and a length of 1000 m.In this step, a 270° C. embossing ring was pressed onto the edges of thefilm for embossing treatment to give a height of 10 μm in order toprevent film adherence in the roll film. Thus, roll film was obtained.

Three cellulose triacetate film samples were taken from the centralportion in the transverse direction of the roll film. Refractive indicesnx, ny, and nz of the samples were measured in the same manner as above,and retardation in plane R0 and retardation in the thickness directionRt of the sample were calculated. In the central portion of the samples,R0 was 2 nm, and Rt was 52 nm. The direction of the delayed phase axisof the samples was lower than the measurable limit, since R0 wasextremely low. Further, the rate of water content of the samples was1.3%.

<<Preparation of Comparative Film 2 (TAC Film 2)>>

Comparative film 2 (TAC film 2) was prepared in the same manner as inTAC film 1, except that the thickness was 41 μm.

Three cellulose triacetate film samples were taken from the centralportion in the transverse direction of the film. Refractive indices nx,ny, and nz of the samples were measured in the same manner as above, andretardation in plane R0 and retardation in the thickness direction Rt ofthe sample were calculated. In the central portion of the samples, R0was 1 nm, and Rt was 31 nm. The direction of the delayed phase axis ofthe samples was lower than the measurable limit, since R0 was extremelylow. Further, the rate of water content of the samples was 3.9%.

<<Measurement of Other Properties of Cellulose Ester Film>>

(Rate of Water Content)

The cellulose ester film prepared above was cut into a 10 cm×10 cm filmsample, subjected to humidity conditioning at 23° C. and at 80% RH for48 hours, and the resulting sample had a weight of W₁. Subsequently, thesample was dried at 120° C. for 45 minutes, and the resulting sample hada weight of W₂. The rate of water content at 23° C. and at 80% RH wasobtained from the following formula:Rate of water content (%)={(W ₁ −W ₂)/W ₂}×100(Number of Luminescent Foreign Materials)

The cellulose ester film prepared above was inserted between twopolarizing plates arranged in a crossed Nicol state to prepare alaminate. When light is projected from one side of the laminate,luminescent foreign materials are observed on the other side of thelaminate as bright materials, employing a microscope. The number per 25mm² of luminescent foreign materials with a size of not less than 5 μmwere measured at ten areas of the laminate. The sum of the number wasdefined as the number per 250 mm² of luminescent foreign materials. Themeasurement was repeated 5 times and the average was defined as thenumber of luminescent foreign materials. The microscope was employedunder conditions of a magnifying power of 30 and transmission light.

Properties of each cellulose ester film are shown in Table 1. In Table1, cellulose ester film is abbreviated simply as film.

TABLE 1 Total Stretch- Acetyl Propionyl acyl Stretch- ing * DryingNumber of substi- substi- substi- ing temper- Residual temper- Rate ofluminescent Film RO Rt tution tution tution magni- ature solvent aturewater foreign No. (nm) (nm) degree degree degree fication (° C.) (%) (°C.) content materials 1 88 205 2.00 0.80 2.80 1.65 110 25 120 1.8 64 247 132 1.90 0.75 2.65 1.40 110 30 125 1.8 48 3 85 210 1.60 1.20 2.801.50 100 25 120 2.1 52 4 93 227 2.00 0.80 2.80 1.55/ 120 20 130 1.6 67MD1.05 5 44 136 1.90 0.75 (butyryl) 2.65 1.40 115 30 120 1.3 51 6 43 1262.00 0.80 2.80 1.55 110 25 120 1.1 29 7 102  163 2.30 0.50 2.80 MD1.70155 12 130 2.0 77 TAC1  2  52 2.92 0   2.92 1.07 — — 120 1.8 65 TAC2  1 31 2.92 0   2.92 1.07 — — 120 3.9 34 * Residual solvent shows one atthe beginning of stretching.(Preparation of Polarizing Plate Sample 1) A 120 μm thick polyvinylalcohol film was immersed in an aqueous solution comprised of 1 part byweight of iodine, 2 parts by weight of potassium iodide, and 4 parts byweight of boric acid, and stretched at 50° C. by a factor of 4 to obtaina polarizing film. Subsequently, cellulose ester film 1 was laminatedonto the polarizing film according to the following procedures to obtaina polarizing plate sample 1.

(1) The cellulose ester film was cut in the form of a rectangle 13 asshown in FIG. 2 to obtain two sheets of cellulose ester film with a sizeof 15 cm×18 cm. The resulting sheets were immersed in an aqueous 2mol/liter sodium hydroxide solution at 60° C. for 1 minute, washed withwater, and dried to obtain two cellulose ester film samples aspolarizing plate protective films.

FIG. 2 is a schematic view of a long length cellulose ester film in theinvention manufactured according to a cast film manufacture process. InFIG. 2, numerical number 10 represents the cellulose ester film in theinvention 10, an arrow 11 is a cast direction in the cast filmmanufacture process, and an arrow 12 is a transverse direction in thecast film manufacture process. The cellulose ester film used for apolarizing plate is for example, a cellulose ester film which is cutinto the form of a rectangle 13, so that the side of the rectangle is inparallel with or normal to the transverse direction.

(2) The polarizing film obtained above was cut to obtain two celluloseester film samples with the same size, and the resulting samples wereimmersed for 1 to 2 seconds in a polyvinyl alcohol adhesive with a solidcontent of 2% by weight.

(3) Excessive adhesive being removed, one of the two cellulose esterfilm samples obtained above was laminated onto one surface of theresulting polarizing film, and the other was laminated onto the othersurface of the resulting polarizing film, as shown in FIG. 3.

FIG. 3 shows a schematic view of the polarizing plate of the invention.The polarizing plate 6 a is a polarizing plate in which a polarizingfilm 2 a is provided between two of the cellulose ester film la in theinvention. The cast direction 3 a of the cellulose ester film 1 a in thecast film manufacture process is in parallel with the stretchingdirection 4 a of the polarizing film 2 a.

(4) Pressure was applied through a hand roller to the laminate with thepolarizing film and the cellulose ester film to remove foams orexcessive adhesive from the ends of the laminate. The pressure appliedby the hand roller was from 20 to 30 N/cm², and the roller speed was 2m/min.

(5) The resulting laminate was dried at 80° C. for 2 minutes in a dryer.Thus, a polarizing plate sample 1 was obtained.

The resulting polarizing plate sample 1 (referred to also as viewingangle increasing polarizing plate sample 1) was incorporated into avertical alignment type (VA type) liquid crystal cell according to themethod described later, and evaluated for viewing angle.

(Preparation of Polarizing Plate Sample 2)

Polarizing plate sample 2 was prepared in the same manner as inpolarizing plate sample 1, except that cellulose ester film 2 was used,and evaluated for viewing angle in the same manner as above.

(Preparation of Polarizing Plate Sample 3)

Polarizing plate sample 3 was prepared in the same manner as inpolarizing plate sample 1, except that cellulose ester film 3 was used,and evaluated for viewing angle in the same manner as above.

(Preparation of Polarizing Plate Sample 4)

Polarizing plate sample 4 was prepared according to the followingprocedures employing comparative film 1 (cellulose triacetate film: TACfilm 1) and cellulose ester film 4.

A 120 μm thick polyvinyl alcohol film was immersed in an aqueoussolution comprised of 1 part by weight of iodine, 2 parts by weight ofpotassium iodide, and 4 parts by weight of boric acid, and stretched at50° C. by a factor of 4 to obtain a polarizing film.

(1) TAC film and cellulose ester film were cut into a rectangle 13 asshown in FIG. 2, respectively, each size being 15 cm×18 cm, and theresulting films were immersed in an aqueous 2 mol/liter sodium hydroxidesolution at 60° C. for 1 minute, washed with water, and dried to obtaina TAC film sample 1 and a cellulose ester film 4 as a polarizing plateprotective film, respectively.

(2) The polarizing film obtained above was cut into the same size as theresulting polarizing plate protective film, and immersed for 1 to 2seconds in a polyvinyl alcohol adhesive with a solid content of 2% byweight.

(3) Excessive adhesive being removed from the polarizing film, the TACfilm sample 1 was laminated onto one surface of the resulting polarizingfilm and the cellulose ester film sample 4 was laminated onto the othersurface of the resulting polarizing film, as shown in FIG. 3.

(4) Pressure was applied through a hand roller to the laminate with thepolarizing film and the cellulose ester film to remove foams orexcessive adhesive from the ends of the laminate. The pressure appliedby the hand roller was from 20 to 30 N/cm², and the roller speed was 2m/min.

(5) The resulting laminate was dried at 80° C. for 2 minutes in a dryer.Thus, a polarizing plate sample 4 was obtained.

The resulting polarizing plate sample 4 (referred to also as viewingangle increasing polarizing plate sample 4) was incorporated into avertical alignment type (VA type) liquid crystal cell according to themethod described later, so that the cellulose ester film 4 faced theglass plate of the liquid crystal cell, and evaluated for viewing angle.

(Preparation of Polarizing Plate Sample 5)

Polarizing plate sample 5 was prepared in the same manner as inpolarizing plate sample 4, except that cellulose ester film 5 was usedinstead of cellulose ester film 4. The resulting polarizing plate sample5 (referred to also as viewing angle increasing polarizing plate sample5) was incorporated into a vertical alignment type (VA type) liquidcrystal cell according to the method described later, so that thecellulose ester film 5 faced the glass plate of the liquid crystal cell,and evaluated for viewing angle.

(Preparation of Polarizing Plate Sample 6)

Polarizing plate sample 6 was prepared in the same manner as inpolarizing plate sample 4, except that cellulose ester film 6 was usedinstead of cellulose ester film 4. The resulting polarizing plate sample6 (referred to also as viewing angle increasing polarizing plate sample6) was incorporated into a vertical alignment type (VA type) liquidcrystal cell according to the method described later, so that thecellulose ester film 6 faced the glass plate of the liquid crystal cell,and evaluated for viewing angle.

(Preparation of Polarizing Plate Sample 7)

Polyvinyl alcohol with an average molecular weight of 3800 and asaponification degree of 99.5 mol % was dissolved in water to prepare a5.0% by weight polyvinyl alcohol solution. The resulting solution wascast onto a polyethylene terephthalate film, dried, and peeled from thepolyethylene terephthalate film to obtain a polyvinyl alcohol film. Thepolyvinyl alcohol film was immersed in an aqueous solution comprised of2 g/liter of iodine and 60 g/liter of potassium iodide at 30° C. for 240seconds, immersed in an aqueous solution comprised of 70 g/liter ofboric acid and 30 g/liter of potassium iodide for 5 minutes for boricacid treatment while stretched by a factor of 4 in the transportingdirection, and dried. Thus, a polarizing film was obtained.

The cellulose ester film 1 obtained above, which had been wound around acore of a glass fiber reinforced resin with a diameter of 200 mm to forma film roll having a width of 1 m and a length of 1000 m, was unwoundfrom the roll, immersed in an aqueous 2 mol/liter sodium hydroxidesolution at 60° C. for 1 minute, washed with water, and dried. Theresulting saponificated cellulose ester film was continuously laminated(roll to roll) as a protective film through a polyvinyl alcohol typeadhesive onto both sides of the polarizing film obtained above. Theresulting laminate was cut into a rectangle 13 with a size of 15 cm×18cm as shown in FIG. 2.

The resulting polarizing plate sample 7 (referred to also as viewingangle increasing polarizing plate sample 7) was evaluated for viewingangle in the same manner as in polarizing plate sample 1.

(Preparation of Comparative Polarizing Plate Sample 8A)

Comparative polarizing plate sample 8A was prepared in polarizing platesample 1, except that comparative film 1 (TAC film 1) was used insteadof the cellulose ester film 1, and evaluated for viewing angle in thesame manner as in polarizing plate sample 1.

(Preparation of Comparative Polarizing Plate Sample 8B)

Comparative polarizing plate sample 8B was prepared in polarizing platesample 1, except that comparative film 2 (TAC film 1) was used insteadof the cellulose ester film 1, and evaluated for viewing angle in thesame manner as in polarizing plate sample 1.

Constitution of the polarizing plate samples obtained above iscollectively shown in Table 2.

TABLE 2 Cellulose ester Cellulose ester film No. provided film No.provided on the surface of on the surface of Polarizing the polarizingfilm the polarizing film plate on the liquid opposite the liquid sampleNo. crystal cell side crystal cell 1 1 1 2 2 2 3 3 3 4 4 TAC 1 5 5 TAC 16 6 TAC 1 7 1 1 8A TAC 1 TAC 1 8B TAC 2 TAC 2<<Preparation of Liquid Crystal Panel>>

A liquid crystal panel sample for measuring viewing angle was preparedas follows:

The polarizing plate of the liquid crystal panel of a 15 inch liquidcrystal display VL-1530S, produced by Fujitsu Co., Ltd., was peeled fromthe liquid crystal cell, and each of polarizing plate samples 1 through7, and comparative polarizing plate samples 8A and 8B was laminated onthe glass plate of the crystal cell with a combination of the polarizingplate samples shown in Table 3. The lamination was carried out so thatthe cellulose ester film in the invention faced the glass plate of theliquid crystal cell and the absorption axis of the sample was inaccordance with that of the polarizing plate before being peeled. Thus,liquid crystal panel samples 1 through 8 were obtained.

The viewing angle of the resulting crystal panel sample was measuredemploying an EZ-Contrast produced by ELDIM Co., Ltd. The viewing anglewas represented by a range of an angle inclined to the direction normalto the plane of the liquid crystal panel, the angle range showing acontrast ratio between white/black display of 10 or more. The resultsare shown in Table 3.

Herein, the viewing angle shows a range of visual filed providing acontrast ratio between white/black display of 10 or more, and isrepresented by an angle providing a contrast ratio between white/blackdisplay of 10 or more, whose is bisected by the line normal to the planeof the liquid crystal panel.

TABLE 3 Arrangement of Viewing angle polarizing plate (degree) sample Inthe Polarizing At upper, oblique Liquid plate Polarizing lower, leftdirection crystal sample No. plate and right inclined at panel on thesample No. portions of a 45° angle sample viewer on the back the panelto the panel Re- No. side light side plane plane marks 1 1 8A 160 (80 +80) 160 (80 + 80) Inv. 2 2 2 160 (80 + 80) 160 (80 + 80) Inv. 3 8A 3 160(80 + 80) 160 (80 + 80) Inv. 4 8B 4 160 (80 + 80) 160 (80 + 80) Inv. 5 55 160 (80 + 80) 160 (80 + 80) Inv. 6 6 6 160 (80 + 80) 160 (80 + 80)Inv. 7 8A 1 160 (80 + 80) 160 (80 + 80) Inv. 8 8B 8A 160 (80 + 80) 120(60 + 60) Comp. Inv.: Invention, Comp.: Comparative

As is apparent from Table 3, the inventive liquid crystal panel samples,employing the polarizing plate of the invention comprising the celluloseester film in the invention, exhibited greatly increased viewing angle.

Example 2

Polarizing plate samples 9 through 14 were prepared according to thefollowing procedures:

<<Preparation of Polarizing Plate Sample 9>>

(Preparation of Cellulose Ester Film TAC-A)

Cellulose ester film TAC-A was prepared according to the followingprocedures:

Cellulose triacetate (with an acetyl substitution degree of 2.92 and aviscosity average polymerization degree of 300) of 100 parts by weight,2 parts by weight of ethylphthalylethyl glycolate, 8.5 parts by weightof triphenyl phosphate, 350 parts by weight of methylene chloride, and50 parts by weight of ethanol were placed into a tightly sealed vessel,and gradually heated to 45° C. in 60 minutes while slowly stirred.Pressure in the vessel was 121 kPa. The resulting solution was filteredemploying Azumi filter paper No. 244, produced by Azumi Roshi Co., Ltd.,and allowed to stand for 24 hours to remove foams in the solution. Thus,a cellulose ester solution was prepared.

Five parts by weight of the cellulose triacetate described above, 3parts by weight of TINUVIN 326 (produced by Ciba Specialty Co., Ltd.), 7parts by weight of TINUVIN 109 (produced by Ciba Specialty Co., Ltd.), 5parts by weight of TINUVIN 171 (produced by Ciba Specialty Co., Ltd.),and 1 part by weight of AEROSIL 200V (produced by NIPPON AEROSIL Co.,Ltd.) were dissolved in a solvent of 90 parts by weight of methylenechloride and 10 parts by weight of ethanol. Thus, a UV absorber solutionwas obtained. One hundred parts by weight of the cellulose estersolution were mixed with 2 parts by weight of the UV absorber solution,and the mixture was stirred with a static mixer to prepare a dope. Thedope of 35° C. was cast on a stainless steel belt support from a diecoater, dried for 1 minute on the stainless steel whose the rear sidewas brought into contact with 35° C. water, and maintained for 15seconds on the stainless steel cooled whose rear side was furtherbrought into contact with a 15° C. chilled water to form a web. Then,the web was peeled the support.

The residual solvent content at the peeling of the web was 80% byweight. Subsequently, employing a tenter for stretching, both edges ofthe peeled web were held with clips and the distance between the clipswas changed so that the web was stretched at 105° C. only in thetransverse direction at a stretching magnification of 1.07. Thestretched web was dried at 130° C. for 10 minutes while transported onthe rollers to prepare cellulose ester film TAC-A with a thickness of 80μm.

Polarizing plate sample 9 was prepared in the same manner as inpolarizing plate sample 1 of Example 1, except that cellulose ester film2 and cellulose ester film TAC-A obtained above were used.

Liquid crystal panel sample 9 was prepared in the same manner as inExample 1, except that polarizing plate sample 9 obtained above(hereinafter referred to as polarizing plate sample 9 before acceleratedaging treatment) was used. The polarizing plate sample 9 was subjectedto accelerated aging treatment in which the sample was allowed to standat 80° C. and 90% RH for 500 hours (hereinafter referred to aspolarizing plate 9 after accelerated aging treatment), and liquidcrystal panel sample 9′ was prepared in the same manner as in Example 1,except that polarizing plate 9 after accelerated aging treatment wasused. Viewing angle of the resulting panel samples was measured in thesame manner as above.

<<Preparation of Polarizing Plate Sample 10>>

Cellulose ester film TAC-B was prepared in the same manner as incellulose ester film TAC-A above, except that the web was stretched inthe transverse direction at a stretching magnification of 1.15.

Polarizing plate sample 10 was prepared in the same manner as inpolarizing plate sample 9 above, except that the cellulose ester filmTAC-B obtained above was used. Liquid crystal panel sample 10 wasprepared in the same manner as in liquid crystal panel sample 9, exceptthat polarizing plate sample 10 obtained above (hereinafter referred toas polarizing plate 10 before accelerated aging treatment) was used. Thepolarizing plate sample 10 was subjected to accelerated aging treatmentin the same manner as above (hereinafter referred to as polarizing plate10 after accelerated aging treatment), and liquid crystal panel sample10′ was prepared in the same manner as above, except that polarizingplate 10 after accelerated aging treatment was used. Viewing angle ofthe resulting panel samples was measured in the same manner as above.

<<Preparation of Polarizing Plate Sample 11>>

Cellulose ester film TAC-C was prepared in the same manner as incellulose ester film TAC-A above, except that the web was stretched inthe transverse direction at a stretching magnification of 1.20.

Polarizing plate sample 11 was prepared in the same manner as inpolarizing plate sample 9 above, except that the cellulose ester filmTAC-C obtained above was used. Liquid crystal panel sample 11 wasprepared in the same manner as in liquid crystal panel sample 9, exceptthat polarizing plate sample 11 obtained above (hereinafter referred toas polarizing plate 11 before accelerated aging treatment) was used. Thepolarizing plate sample 11 was subjected to accelerated aging treatmentin the same manner as above (hereinafter referred to as polarizing plate11 after accelerated aging treatment), and liquid crystal panel sample11′ was prepared in the same manner as above, except that polarizingplate 11 after accelerated aging treatment was used. Viewing angle ofthe resulting panel samples was measured in the same manner as above.

<<Preparation of Polarizing Plate Sample 12>>

Cellulose ester film TAC-D was prepared in the same manner as incellulose ester film TAC-A above, except that the web was stretched inthe transverse direction at a stretching magnification of 1.02.

Polarizing plate sample 12 was prepared in the same manner as inpolarizing plate sample 9 above, except that the cellulose ester filmTAC-D obtained above was used. Liquid crystal panel sample 12 wasprepared in the same manner as in liquid crystal panel sample 9, exceptthat polarizing plate sample 12 obtained above (hereinafter referred toas polarizing plate 12 before accelerated aging treatment) was used. Thepolarizing plate sample 12 was subjected to accelerated aging treatmentin the same manner as above (hereinafter referred to as polarizing plate12 after accelerated aging treatment), and liquid crystal panel sample12′ was prepared in the same manner as above, except that polarizingplate 12 after accelerated aging treatment was used. Viewing angle ofthe resulting panel samples was measured in the same manner as above.

<<Preparation of Polarizing Plate Sample 13>>

Polarizing plate sample 13 was prepared in the same manner as inpolarizing plate sample 9 above, except that the cellulose ester filmTAC-A was turned 90° in plane and laminated onto the polarizing film.Liquid crystal panel sample 13 was prepared in the same manner as inliquid crystal panel sample 9, except that polarizing plate sample 13obtained above (hereinafter referred to as polarizing plate 13 beforeaccelerated aging treatment) was used. The polarizing plate sample 13was subjected to accelerated aging treatment in the same manner as above(hereinafter referred to as polarizing plate 13 after accelerated agingtreatment), and liquid crystal panel sample 13′ was prepared in the samemanner as above, except that polarizing plate 13 after accelerated agingtreatment was used. Viewing angle of the resulting panel samples wasmeasured in the same manner as above.

<<Preparation of Polarizing Plate Sample 14>>

Cellulose ester film TAC-E was prepared in the same manner as incellulose ester film TAC-A above, except that the web was not stretchedin the transverse direction.

Polarizing plate sample 14 was prepared in the same manner as inpolarizing plate sample 9 above, except that the cellulose ester filmTAC-E obtained above was used. Liquid crystal panel sample 14 wasprepared in the same manner as in liquid crystal panel sample 9, exceptthat polarizing plate sample 14 obtained above (hereinafter referred toas polarizing plate 14 before accelerated aging treatment) was used. Thepolarizing plate sample 14 was subjected to accelerated aging treatmentin the same manner as above (hereinafter referred to as polarizing plate14 after accelerated aging treatment), and liquid crystal panel sample14′ was prepared in the same manner as above, except that polarizingplate 14 after accelerated aging treatment was used. Viewing angle ofthe resulting panel samples was measured in the same manner as above.

The results are shown in Table 4.

TABLE 4 Cellulose Cellulose ester film ester film No. No. provided onLiquid provided on the the surface of crystal surface of the thepolarizing Viewing panel Polarizing polarizing film film opposite anglesample plate on the liquid the liquid (degree) No. sample No. crystalcell side crystal cell L R U L Remarks  9 9 2 TAC-A 64 64 28 27 No  9′ 92 TAC-A 64 64 27 28 deteri- oration 10 10 2 TAC-B 65 65 27 27 No 10′ 102 TAC-B 65 65 28 26 deteri- oration 11 11 2 TAC-C 65 65 27 27 Slight 11′11 2 TAC-C 61 60 28 25 deteri- oration 12 12 2 TAC-D 65 65 27 27 Slight12′ 12 2 TAC-D 62 62 25 27 deteri- oration 13 13 2 TAC-A (MD) 65 65 2727 Little 13′ 13 2 TAC-A (MD) 65 65 28 26 deteri- oration 14 14 2 TAC-E)65 65 27 27 A small 14′ 14 2 TAC-E 55 57 23 23 Deteri- oration L: Leftportion; R: Right portion; U: Upper portion; L: Lower portion

As is apparent from Table 4, the polarizing plate samples 9 through 14comprising the cellulose ester film in the invention provided greatlyincreased viewing angle.

It has been confirmed that the polarizing plate in the inventioncomprising cellulose ester film (provided on the surface of thepolarizing film opposite the glass plate of the liquid crystal cell),which does not have a phase difference function but is stretched at aspecific stretching magnification, provides improved dimensionalstability.

The polarizing plate samples 9 through 14 comprised cellulose ester film2, which had been stretched at a stretching magnification of 1.40, onone surface of the polarizing film. Curling of the polarizing platesamples 9 through 14 after the accelerated aging treatment was visuallyevaluated. As a result, curling due to dimensional change was observedin the polarizing plate sample 14 comprising TAC-E, which had not beenstretched. Curling was reduced in the polarizing plate sample comprisingTAC-D, which had been stretched at a stretching percentage of 2%, or inthe polarizing plate sample comprising TAC-C, which had been stretchedat a stretching percentage of 20%. Curling was scarcely observed in thepolarizing plate sample comprising TAC-B, which had been stretched at astretching percentage of 15%, or in the polarizing plate samplecomprising TAC-A, which had been stretched at a stretching percentage of7%.

[Effect of the Invention]

The present invention can provide a polarizing plate having highresistance to heat or humidity, high strength, and minimized foreignmaterials, and giving wide viewing angle, and can provide a liquidcrystal display employing the polarizing plate.

1. A polarizing plate used in a vertical alignment mode liquid crystal display comprising a liquid crystal cell, the polarizing plate comprising a polarizing film and a mixed fatty acid cellulose ester film with optical biaxiality provided between the polarizing film and the liquid crystal cell, the mixed fatty acid cellulose ester of the mixed fatty acid cellulose ester film having a total acyl substitution degree of from more than 1.5 to 2.8 and an acetyl substitution degree of less than 2.0, wherein the mixed fatty acid cellulose ester film has a retardation in plane R0 of from 31 to 120 nm, and a retardation in the thickness direction Rt of from 60 to 300 nm, in which R0 is represented by the following formula (1) and Rt is represented by the following formula (2): R0=(nx−ny)×d  formula (1) Rt={(nx+ny)/2−nz}×d  formula (2) wherein nx is refractive index in plane of the film in a first direction giving a maximum refractive index, ny is refractive index in plane of the film in a second direction normal to the first direction, nz is refractive index in the thickness direction of the film, and d is thickness (nm) of the film.
 2. The polarizing plate of claim 1, wherein the mixed fatty acid cellulose ester film has a retardation in plane R0 of from 31 to 60 nm, and a retardation in the thickness direction Rt of from 90 to 200 nm.
 3. The polarizing plate of claim 2, wherein the mixed fatty acid cellulose ester film has a retardation in plane R0 of from 31 to 50 nm, and a retardation in the thickness direction Rt of from 110 to 150 nm.
 4. The polarizing plate of claim 1, wherein the mixed fatty acid cellulose of the mixed fatty acid cellulose ester film has an acetyl group and a propionyl group in the ester bond.
 5. The polarizing plate of claim 4, wherein the mixed fatty acid cellulose ester has a total acyl substitution degree of from 1.5 to 2.3, and a propionyl substitution degree of from 0.6 to 1.2.
 6. The polarizing plate of claim 1, wherein the mixed fatty acid cellulose ester of the mixed fatty acid cellulose ester film has an acetyl group and a butyryl group in the ester bond.
 7. The polarizing plate of claim 1, wherein the mixed fatty acid cellulose ester film has a thickness of from 30 to 110 μm.
 8. The polarizing plate of claim 1, wherein when the mixed fatty acid cellulose ester film is arranged in a crossed Nicol state, and luminescent foreign materials are observed, the number of luminescent foreign materials with a size of from 5 to 50 μm is not more than 150 per 250 mm² of the film, and the number of luminescent foreign materials with a size exceeding 50 μm is zero per 250 mm² of the film.
 9. The polarizing plate of claim 1, wherein the mixed fatty acid cellulose ester film is manufactured by a process comprising the steps of giving optical biaxiality to a long length mixed fatty acid cellulose ester film, and winding the resulting film around a spool to obtain a long length roll film.
 10. The polarizing plate of claim 1, wherein the polarizing plate is manufactured by a process comprising the steps of: giving optical biaxiality to a long length mixed fatty acid cellulose ester film; treating the film to give a maximum refractive index in the transverse direction of the film; winding the resulting film around a spool to obtain a roll film; providing a polarizing film containing a dichroic substance; and laminating the polarizing film with the roll film.
 11. The polarizing plate of claim 10, wherein the optical biaxiality giving step is a step in which the long length mixed fatty acid cellulose ester film is stretched in the transverse direction of the film when it has a residual solvent content of not less than 10% by weight during manufacture.
 12. The polarizing plate of claim 1, wherein the mixed fatty acid cellulose ester film contains 1 to 30 parts by weight of at least one plasticizer selected from the group consisting of a phosphate compound, a fatty acid ester compound, a citrate compound, and a phthalate compound based on 100 parts by weight of the cellulose ester.
 13. The polarizing plate of claim 1, wherein the mixed fatty acid cellulose ester film contains 0.005 to 0.3 parts by weight of particles having an average particle size of not more than 0.1 μm, based on 100 parts by weight of the cellulose ester.
 14. The polarizing plate of claim 1, wherein the mixed fatty acid cellulose ester film contains 0.8 to 3.0 parts by weight of a UV absorber based on 100 parts by weight of the cellulose ester.
 15. A polarizing plate used in a vertical alignment mode liquid crystal display comprising a liquid crystal cell, the polarizing plate comprising a polarizing film and a mixed fatty acid cellulose ester film with optical biaxiality provided between the polarizing film and the liquid crystal cell, the mixed fatty acid cellulose ester film being provided on a first surface of the polarizing film, a cellulose ester film being provided on a second surface of the polarizing film opposite the mixed fatty acid cellulose ester film, and the stretching direction of the mixed fatty acid cellulose ester film being in accordance with that of the cellulose ester film, wherein the mixed fatty acid cellulose ester film has a retardation in plane R0 of from 31 to 120 nm, and a retardation in the thickness direction Rt of from 60 to 300 nm, in which R0 is represented by the following formula (1) and Rt is represented by the following formula (2): R0=(nx−ny)×d  formula (1) Rt{(nx+ny)/2−nz}×d  formula (2) wherein nx is refractive index in plane of the film in a first direction giving a maximum refractive index, ny is refractive index in plane of the film in a second direction normal to the first direction, nz is refractive index in the thickness direction of the film, and d is thickness (nm) of the film.
 16. The polarizing plate of claim 15, wherein cellulose triacetate film is provided on the surface of the polarizing film opposite the mixed fatty acid cellulose ester film.
 17. The polarizing plate of claim 15, wherein the angle formed between the orientation direction of a dichroic substance in the polarizing film and the direction giving a maximum refractive index in plane of the mixed fatty acid cellulose ester film is in the range of from 80° to 100°.
 18. The polarizing plate of claim 15, wherein the angle formed between the orientation direction of a dichroic substance in the polarizing film and the direction giving a maximum refractive index in plane of the mixed fatty acid cellulose ester film is in the range of from 0 to 10°.
 19. The polarizing plate of claim 15, wherein the cellulose ester film provided on the second surface of the polarizing film has been stretched in the transverse direction during or after manufacture.
 20. The polarizing plate of claim 19, wherein the cellulose ester film provided on the second surface of the polarizing film has been stretched at a stretching magnification of from 1.01 to 1.2.
 21. The polarizing plate of claim 20, wherein the cellulose ester film provided on the second surface of the polarizing film has been stretched at a stretching magnification of from 1.05 to 1.15. 